If you're tired of manually pouring freshwater into your tank every morning, setting up an automatic top off reservoir is probably the best gift you can give yourself. It's one of those "quality of life" upgrades that you don't realize you need until you finally have it. Suddenly, you aren't tethered to your aquarium every single day just to keep the salinity stable. But, as simple as a container of water sounds, there are actually a few things that can go wrong if you don't put a bit of thought into the setup.
I remember when I first started out, I thought I could just use any old plastic bin I had lying around. While that technically works, I quickly learned that the "reservoir" part of an ATO system is just as important as the pump or the sensors. If it's too small, you're still filling it up all the time. If it's not covered, you're dealing with dust and hair getting into your clean RO/DI water. It's all about finding that sweet spot between convenience and aesthetics.
Why the Size of Your Reservoir Really Matters
When you're looking at an automatic top off reservoir, the first question is usually: how big does it need to be? There isn't a one-size-fits-all answer, but a good rule of thumb is to aim for about a week's worth of evaporation. If you have a 50-gallon tank and it loses half a gallon a day, a five-gallon reservoir is perfect. It gives you some wiggle room so you aren't constantly checking the water level.
However, you don't want to go too big if you don't have the space. A massive 20-gallon drum sitting next to a small nano tank looks a bit ridiculous, and it can actually be a safety hazard. If your ATO sensor ever fails in the "on" position, the pump will dump the entire reservoir into your tank. If that reservoir is huge, it'll tank your salinity and potentially flood your living room. It's always a balance between convenience and minimizing the damage if something goes sideways.
Finding the Perfect Spot for Your Setup
Most people want to hide their automatic top off reservoir inside the aquarium stand. It keeps things looking clean and professional. But let's be honest, cabinet space is usually at a premium. Between the sump, the skimmer, and the mountain of cables we all seem to accumulate, there isn't much room left for a water tank.
This is where those skinny, "space-saver" reservoirs come in handy. They're designed to be tall and thin so they can slide into that awkward gap between the sump and the side of the stand. If you really can't fit anything inside the cabinet, you might have to get creative. I've seen people use decorative cabinets or even end tables next to the tank to hide a larger reservoir. If it's going to be out in the open, you'll probably want something a bit more attractive than a white bucket from the hardware store.
Material Choices: Glass, Acrylic, or Plastic?
You'll find reservoirs made out of pretty much anything that can hold water. Acrylic is the gold standard for most hobbyists because it's lightweight, looks great, and is easy to see through. It's nice to be able to glance at the side of the tank and see exactly how much water you have left without having to take a lid off.
Glass is another solid option, and it's often cheaper if you're just using a small standard aquarium as your reservoir. The downside is that glass is heavy and a bit more prone to cracking if you're moving it around a lot for cleaning.
Then there's the budget-friendly plastic bin or food-grade bucket. Honestly, there's no shame in using a $5 bucket as your automatic top off reservoir. It does the job perfectly fine. Just make sure the plastic is food-safe (look for the BPA-free or "5" recycling symbol) so it doesn't leach any weird chemicals into your top-off water. The only real downside here is the "ugly factor" and the fact that you can't usually see the water level from across the room.
Avoiding the Infamous Siphon Effect
This is the part where a lot of people accidentally flood their floors. If your automatic top off reservoir is sitting higher than the end of the top-off hose (where the water enters the sump), you're going to run into a siphon issue. Once the pump starts, the water will keep flowing even after the pump turns off because gravity takes over.
I've seen this happen more times than I can count. The fix is actually pretty simple: make sure the end of your output hose is always higher than the highest water level in your reservoir. You also want to make sure the hose isn't submerged in the sump water, or you'll end up siphoning saltwater back into your freshwater reservoir when the pump stops. It's a messy mistake that's easily avoided with a little bit of positioning.
Keeping Things Clean and Functional
Even though you're only putting "pure" RO/DI water into your reservoir, it won't stay perfectly clean forever. Over time, you might notice a bit of a slimy film developing on the walls or the pump. This is usually just biofilm or a bit of bacterial growth. It's a good idea to completely empty and wipe down your automatic top off reservoir every few months.
Another tip is to keep a lid on it. It might seem like a small thing, but a lid prevents evaporation from the reservoir itself and keeps out dust, pet hair, and whatever else is floating around your house. If you're using a bucket, just drill a small hole in the lid for your hose and power cord. If you bought a fancy acrylic one, it should already come with a nice fitted top.
Integrating Sensors and Alarms
Most modern ATO systems come with sensors that sit in your sump, but some people like to add a second sensor inside the reservoir itself. This is a total lifesaver because it can trigger an alarm or shut off the pump when the reservoir is nearly empty. Running a small ATO pump dry for a day or two is a one-way ticket to a burnt-out motor.
I'm a big fan of anything that gives me peace of mind. If you're heading out for a weekend trip, knowing that your system won't burn itself out or stop working because it ran out of water is worth the extra few bucks for a low-level sensor. Some high-end reservoirs even have built-in float valves, so you can hook them directly up to your RO/DI system, but that's a whole different level of complexity (and potential flood risk) that most people don't really need.
The DIY vs. Retail Debate
Should you buy a dedicated automatic top off reservoir or just make one? If you're on a budget, DIY is the way to go. You can find plenty of slim plastic containers at big-box stores that fit perfectly in tight spaces. A little bit of drilling and some bulkheads, and you've got a functional reservoir for a fraction of the price.
On the other hand, the retail versions are often built with features that are hard to replicate at home. We're talking about built-in graduated markings (so you know exactly how many gallons are left), dedicated spots for sensors, and sleek designs that match the rest of your gear. If your tank is in a high-traffic area like a living room, sometimes it's worth paying a little extra for something that doesn't look like a science experiment.
At the end of the day, the best reservoir is the one that fits your space and keeps your tank's salinity rock-steady. Once you get it dialed in, you'll wonder how you ever lived without it. It turns one of the most tedious parts of reef-keeping into a set-it-and-forget-it task, leaving you more time to actually enjoy the fish and corals you're working so hard to keep alive.