Here’s a question most people in Los Cabos never think to ask: which is dirtier — your cistern or your tinaco?
If you guessed the cistern — the dark, underground tank you’ve never opened — you’d be wrong. In the majority of homes we’ve seen across Cabo San Lucas and San José del Cabo, the tinaco on the roof is in worse condition than the cistern below. It’s more contaminated, more degraded, and more neglected. And it’s the last thing your water touches before it reaches your shower, your kitchen faucet, and your mouth.
The cistern gets all the worry. The tinaco gets none. That’s backwards.
Tinaco vs. Cisterna: What’s the Difference?
A cisterna (cistern) is your primary underground water storage tank — typically 5,000 to 20,000 liters of reinforced concrete or polyethylene, filled by pipa truck delivery or intermittent municipal supply. A tinaco is the smaller rooftop tank — typically 1,100 liters in the iconic black Rotoplas — that gravity-feeds your household plumbing. They work as a team: the cistern stores the bulk supply, an electric pump pushes water up to the tinaco, and the tinaco provides the gravity pressure your taps need. Together, they form the complete water system in every Cabo home. But they face completely different threats, and the one most people ignore is usually the one in worse shape.
Why This Confusion Matters in Cabo
In much of Mexico, people use “cisterna” and “tinaco” interchangeably, or refer to both as “el tanque.” Among English-speaking expats and vacation rental guests, the confusion is even deeper — many don’t realize they have two tanks at all. They know water comes from somewhere, and they’ve heard the word “cistern,” and that’s the extent of it.
This matters because maintenance gets missed. A property manager says “the cistern was cleaned” — but did they also clean the tinaco? A homeowner hires a cleaning service — but the service only did the underground tank because that’s what “cistern cleaning” typically means. Meanwhile the tinaco on the roof, baking in 40°C desert sun twelve months a year, hasn’t been opened in three years.
In Cabo’s climate, that distinction between “cleaned the cistern” and “cleaned the whole system” is the difference between solving the problem and missing the last — and often worst — stage of contamination.
How They Compare: A Side-by-Side
Location and environment. Your cistern is underground — insulated by earth, protected from UV radiation, naturally cool. Ground temperature in Los Cabos stays roughly 22–28°C year-round. Your tinaco is on the roof — fully exposed to direct desert sun, ambient temperatures regularly exceeding 40°C in summer, UV radiation degrading the plastic every day. The water inside a rooftop tinaco can reach 35–40°C on a hot afternoon.
Capacity. Cisterns hold your strategic reserve: 5,000 to 20,000 liters, enough for one to three weeks between deliveries. Tinacos hold your daily operational supply: typically 1,100 liters, sometimes 2,500 liters, enough for roughly one day of normal household use.
Material. Cisterns are usually reinforced concrete (older construction) or large polyethylene tanks (newer builds). Concrete is durable but porous — it can harbor biofilm in surface irregularities. Tinacos are almost universally black polyethylene (Rotoplas is the dominant brand in Mexico). The black color absorbs heat aggressively. The plastic degrades under UV over 10–15 years, becoming brittle, cracking, and eventually failing.
Water residence time. Water in your cistern might sit for days to weeks — raising water age concerns. Water in your tinaco cycles faster — typically less than 24 hours in an actively used home. But here’s the catch: the tinaco’s higher temperature accelerates bacterial growth, so even short residence time at 35°C can support rapid microbial multiplication. The chlorine decay rate at 35°C is roughly double what it is at 22°C.
Contamination sources. The cistern’s contamination comes primarily from sediment delivered by pipas, biofilm growth on walls, and whatever enters through the fill port. The tinaco’s contamination comes from heat-driven bacterial proliferation, UV-degraded material leaching, algae growth when light enters through a cracked lid, and physical contaminants — insects, bird droppings, leaves, dust — that enter through gaps. We’ve found dead birds in tinacos. Wasps nesting under cracked lids. Gecko droppings coating the interior walls. This doesn’t happen in a sealed underground cistern.
Why the Tinaco Is Usually the Bigger Problem
The cistern gets the attention because it’s mysterious. It’s underground. It’s dark. People imagine the worst. The tinaco, by contrast, is visible — right there on the roof, in plain sight. And that familiarity breeds complacency.
But consider what the tinaco actually endures:
Year-round UV assault. Cabo gets approximately 300+ days of sunshine per year. A black polyethylene tank absorbs UV radiation continuously. Over 8–12 years, the material becomes brittle, develops micro-cracks, and can fail catastrophically — splitting open and flooding whatever is below. Long before it fails, the degraded surface becomes rougher at the microscopic level, providing more attachment points for biofilm.
Extreme temperature cycling. Every day, the tinaco heats up in the sun and cools down at night. This thermal cycling stresses the material and the connections (pipe joints, lid seals). It also creates conditions that promote bacterial growth — warm water with depleted chlorine is an ideal incubation environment.
The lid problem. Tinaco lids are the weakest link. They crack, warp, blow off in wind, or simply don’t seat properly. A tinaco with a compromised lid is an open-air container. Insects enter. Dust and organic matter blow in. In rare cases, small animals fall in and decompose. If light enters, algae grows on the interior walls — green slimy growth that contaminates your water with taste, odor, and a bacterial food source.
It’s the last stage before your tap. This is the key point most people miss. Even if your cistern is perfectly maintained and your water arrives pristine from the pipa — if the tinaco is contaminated, that’s what you shower in, brush your teeth with, and wash your dishes in. The tinaco is the last point of contamination risk before consumption. Whatever is in the tinaco is in your water.
Nobody cleans it. When people hire a “cistern cleaning” service, the crew typically drains, scrubs, and disinfects the underground cistern. Many don’t touch the tinaco — because it’s on the roof, harder to access, and not what the customer asked for. Ask explicitly. Always.
What the Cistern Does Better
This isn’t to say your cistern doesn’t matter. It does — enormously. It’s the larger reservoir, the first point of contact for delivered water, and where sediment accumulates in the greatest quantities.
But the cistern has natural advantages the tinaco doesn’t:
Temperature stability. Underground, insulated by earth, your cistern stays 15–20 degrees cooler than the tinaco. Cooler water means slower bacterial growth and slower chlorine decay. Your cistern water may hold residual chlorine for 3–5 days; in the tinaco at roof temperature, it might be gone within hours.
UV protection. Zero UV exposure means zero material degradation from sunlight. A concrete cistern can last 30+ years with minimal maintenance. The structure doesn’t weaken over time the way polyethylene does in direct sun.
Sealed environment. A properly maintained cistern with an intact lid, screened vent, and sealed overflow is a relatively closed system. Contaminants don’t blow in. Animals can’t enter. Light doesn’t promote algae. The cistern’s vulnerability is what’s delivered into it (pipa water quality, sediment), not what falls into it from the environment.
Larger volume distributes risk. A contaminant introduced into a 10,000L cistern is diluted across that entire volume. The same contaminant in a 1,100L tinaco is ten times more concentrated.
The Connection Most People Miss
Here’s the systems-level insight: your cistern and tinaco aren’t separate storage containers. They’re stages in a pipeline, and water quality at your tap is determined by the weakest stage.
Think of it like a kitchen: it doesn’t matter how carefully you source your ingredients if you store them on a dirty counter and serve them on unwashed plates. A perfectly clean cistern feeding water into a contaminated tinaco delivers contaminated water to your taps. The tinaco is the dirty plate.
This also connects to the chlorine decay curve. Residual chlorine from the pipa delivery or municipal treatment provides protection — but it’s a decaying protection. By the time water has sat in the cistern for days and then transferred to a hot tinaco, the chlorine is likely at or near zero. The tinaco is where your water sits unprotected at elevated temperature, right before you use it.
For people in condos, this gets even more complex. Your building might have a well-maintained shared cistern — but each unit typically has its own tinaco, and maintenance of individual tinacos varies wildly from unit to unit.
What to Do About It
The free fix: Go look at your tinaco. Right now, or this weekend. You don’t need to climb the roof (though if you can safely access it, do). Look up from ground level. Is the lid intact? Is it properly seated? Do you see cracks in the tank body? Is it the original tank from when the house was built? If the tank is visibly cracked, discolored in patches, or the lid is missing or broken, you’ve identified an immediate issue.
The cheap fix ($200–500 MXN): Replace the tinaco lid if it’s cracked or warped. Buy a proper replacement from any ferretería that stocks Rotoplas parts. The lid should seat snugly with no gaps. This alone eliminates the largest contamination pathway — entry of insects, debris, and light. While you’re up there, check the float valve. If the tinaco is overflowing, the valve needs replacement (about $150–250 MXN for the part).
The right fix ($1,500–5,000 MXN): Have both your cistern AND tinaco cleaned together. Insist that the cleaning service includes the tinaco — many don’t unless you specify. If your tinaco is more than 10 years old and showing signs of UV degradation (brittleness, cracking, discoloration), replace it entirely. A new 1,100L Rotoplas tinaco costs approximately current Rotoplas 1100L price, expected $2,500–4,000 MXN. Source: local ferretería/Home Depot survey. Update quarterly.. Consider upgrading to a light-colored or UV-protected model if available.
The Economics
A new tinaco is one of the cheapest major components in your water system. At price range for a standard 1,100L Rotoplas, it’s significantly less than a pump replacement, a water heater burned out by sediment, or a single emergency pipa delivery.
Tinaco neglect costs you in invisible ways: accelerated biofilm growth that recontaminates your cistern-cleaned water, filter clogging from algae and debris that entered through a broken lid, and skin/hair irritation from bathing in warm, bacterially active water — symptoms most people blame on “Cabo water” rather than their neglected rooftop tank.
The most cost-effective maintenance schedule: clean both cistern and tinaco together every 6–12 months, inspect the tinaco lid and float valve quarterly, and plan for full tinaco replacement every 10–12 years.
Frequently Asked Questions
Can I just use one tank instead of two? ¿Puedo usar solo un tanque en vez de dos? Some newer homes use a pressurized system that pumps directly from the cistern to household lines, eliminating the tinaco. This works but requires a more powerful pump, a pressure tank, and means you lose water pressure during power outages (whereas a tinaco provides gravity-fed flow without electricity). Most Cabo homes still use the traditional two-tank system.
How often should I clean my tinaco? ¿Cada cuánto debo limpiar mi tinaco? Every 6 to 12 months — the same frequency recommended for the cistern. Always clean both at the same time. In Cabo’s heat, algae and biofilm grow faster on the tinaco than the cistern, so if you’re only cleaning one annually, the tinaco arguably needs it more.
My tinaco lid is missing. How urgent is this? Se perdió la tapa de mi tinaco. ¿Qué tan urgente es? Very urgent. An open tinaco is exposed to insects, birds, dust, organic debris, and — critically — sunlight that drives algae growth. Replace it this week. In the meantime, cover the opening with a weighted piece of plywood or similar material. Don’t leave it open.
Should I paint my tinaco white to reduce heat? ¿Debo pintar mi tinaco de blanco para reducir el calor? A white or light-colored tinaco does stay cooler than a black one, reducing internal water temperature by several degrees. However, don’t paint an existing black Rotoplas — the paint may not adhere well and can flake into the water. If you’re replacing your tinaco, look for lighter-colored models or models marketed as UV-resistant.
Is the green stuff in my tinaco dangerous? ¿Lo verde en mi tinaco es peligroso? The green growth is algae, which itself isn’t highly dangerous but indicates that light is entering the tank (lid problem) and that conditions support biological growth. Algae serves as a food source for bacteria, contributes to biofilm formation, and degrades water taste and odor. Clean the tinaco, fix the lid, and it won’t come back.
Related Reading
Want to understand the full system? Start with What Is a Cistern and How Does It Work? — the complete overview of how cistern, pump, and tinaco work together.
Concerned about what’s growing in your tanks? Biofilm: The Invisible Colony in Your Water Tank explains the microbial communities that develop on tank walls and why chemical treatment alone doesn’t solve it.
Ready to schedule cleaning? Make sure you hire a service that includes the tinaco. How to Clean a Cistern covers what to expect and what to demand.
Check Your Tinaco
When was the last time anyone looked at your rooftop tank? The Water Health Diagnostic includes tinaco condition in its risk assessment — take 3 minutes and find out if your system’s weakest link is above your head.