For the 140 gallon system, we have been undecided about a freshwater top off system because we were undecided about adding an external refugium. Initially, we installed a 60 gallon glass cube as the sump tank with the intention of converting to an external display refugium at a later date.
Every few days we would make-up a few gallons of RODI water and add it to the sump. Note again that we don’t worry about holding the salinity perfectly steady. 1.023 to 1.025 is our target range. If it drifts a bit, that’s OK.
In the mid-term, we have decided to keep the 60 gallon glass tank (intended initially for an external refugium) as a sump in the cabinet. And add a new 20 gallon framed glass tank to hold freshwater for top off.
We added a new 770 gph pump to move freshwater to the sump as needed manually by flipping an on/off switch on a power strip for a few seconds.
No auto-top off at this time. The 20 gallon can be re-purposed if needed, and the new water pump can serve in a number of ways including a backup to the return pump should it fail.
Why? An unnecessary expense at the moment. We are STILL undecided about which auto top off system to buy, and we don’t want to do the market research at this time. We don’t like to leave water running under pressure. Note that in our system design, even if the auto top off failed and dumped the entire amount of freshwater into the sump, the sump would not over flow (we are using 25 gallons of the 60 gallon capacity) and the overall salinity change would not be too drastic.
So what have we accomplished. Fewer freshwater makeups and trips from the kitchen to the aquarium. A more consistent water level in the sump for the skimmer. An easier way to more frequently top off. A 20 gallon tank added to the system that could also be used for isolation or quarantine for example. A new water pump that will serve as a backup to the return pump. A new water pump that can be used now in the salt water change process.
Costs: 20 gallon glass tank – $40. 770 gph water pump – $25.