Parameter After Top Off Calculator

Every day, some amount of water will evaporate from surface of your tank. That water contain dissolved minerals which remain in tank after evaporation. As more water evaporate into air, the mineral concentration goes up. Unless you measure it, you don’t know how bad it gets. And if you do, numbers look worse then they feel. Suddenly, you’re concerned that your shrimps requires soft water or that your corals is suffering from stress. In reality, it’s just physics at work on a closed system.

The calculator above do the math for you. All you have to do is input your source water parameter and net water volume. No guesswork required for safety limits or how much to dilute.

Why Your Tank Water Changes

Many keepers think that replacing lost water return things to how they were. Not so much. Adding straight, pure reverse osmosis deionized water to your saltwater tank will drop salinity. That’s because you’re adding fresh water, which lowers it. So you’re not at whatever salinity was there before it started going up due to evaporation. Instead, you’ll be below what it was when it began rising. That means you’re bouncing up and down rather than maintaining a constant value.

The calculator allow you to see how high the water rose before you topped it off and where it settles once it stabilizes again. Sounds trivial? But it can make difference if you have sensitive livestock.

When considering inputs, you really should of check the volume of your system honestly. A common approach is to use the rated capacity of your tank to make the calculation. No! The actual net water volume are significantly lower than the rated capacity of your tank. Consider the displacement of sand, the porosity of your live rock, and where your sump is set at its operating level. If you use the total tank size rather then the true volume of water, the effect of top-off water appear less dramatic. You end up with wider swings in salinity than those models predict. Why? Because there isn’t as much water to buffer the change.

For those of us that maintain fresh water tanks containing species like Caridina and/or Neocaridina shrimp, things changes. Hardness and total dissolved solids (TDS) is now the things to watch rather than salinity. As minerals, they concentrate through evaporation much like salt. To accurately top off a tank using distilled water, you must know how much distilled water to add to achieve your desired TDS range. Here’s where the calculator come into play. It tells you if you should add pure water or mix in some harder source water.

Timing is another consideration. There’s a lag time between the moment evaporation occurs and the top-off kicks on. This means there will be a short period of higher parameters. That’s often enough to cause sensitive coral polyps to pull back even though the overall average may appear just fine. Taking this lag into account, you’re able to better understand what’s going on inside your system. If your automatic top-off is kicking on too late, you’ll know. If your sump reservoir isn’t large enough to help buffer the sudden increase then you’ll know it as well.

Also consider safety caps when you make water changes. Whether you’re changing water up or down, drastic osmotic pressure change will still shock livestock. The chart on the page provides reasonable parameters where one adjustment shouldn’t harm livestock. These limits depend on their sensitivity. Often times it’s safer to make smaller adjustments more frequently rather than big weekly fixes. For example, you may be tempted to add a gallon of RO water to a nano reef to bring down high salinity. This can bleach the tissue of SPS coral or even kill off sensitive zoanthids.

Also at play is testing accuracy. Do you have a refractometer or TDS meter with a large tolerance band? Chasing decimal points isn’t going to do anything for you there. You’ll use that tool as a way to account for differences among equipment. So when you’re tempted to overcorrect on what may be nothing more than measurement noise, it will give you a reality check against the lure of false precision.

Stable parameters are not absolute; they’re more of a pendulum than a target. Each system will have an equilibrium point. Equilibrium depends on both source water quality and evaporation rates. When you understand this, you stop reacting to spikes. Instead, you start preventing them. You begin to view your tank as a changing volume that requires careful balance. Water evaporates, minerals concentrate, and the cycle continues… until you step in with careful accurracy.