A plastic float valve is used in the sump for the addition of evaporation top up water. The valve is located within the sump section containing the return pump inlet. This means that any change in water volume in the entire system will cause a change of the water level in this section, providing a very accurate way of maintaining a constant system salinity. However, the salinity still needs to be monitored as salinity can still vary due to loss of salt in the protein skimmer skimate and salt creep. Connected to the float valve is a 25 litre plastic drum, which actually holds nearly 30 litres of water. Depending on the temperature and humidity, this can be sufficient to last for 2 days to 1 week.

Water drum connected to the float valve, on the left hand side of the sump.

The float valve itself is made totally from plastic and a rubber seal. It is one that is normally used in cooling towers and came form a plumbing and irrigation store for around $20. The inlet is a threaded pipe, so it is mounted though the side of the sump in the same manner as a bulkhead fitting. The float ball is attached to the valve arm by a short length of string, the length of which can be changed depending on the water height that wishes to be maintained.

Float valve installed through the side of the sump.

The water used for evaporation top up is from a DIY deionisation unit that filters the tap water. First stage activated carbon, second stage cationic resin and final stage anionic resin. The deionisation resins were obtained from around 9 Aquarium Pharmaceutical Tap Water Purifier units. The cationic resin changes colour from green to purple when it is exhausted, so it provided a simple visual indication of when the resin needs to be recharged. The great thing about using this type of water purification is that the resin can be recharged, so the cost of this type of water filtration is significantly lower than reverse osmosis. Additionally, water isn't wasted, all water that passes through the unit can be used, unlike reverse osmosis units.

Deionisation filter unit used to purify tap water.

In all the time it has been in operation since 1997, it has run pretty much totally hassle free, with the exception of one occassion, which will be covered in a moment. In probably about 5 instances it has stayed in the close position even though the water level has dropped. All that was involved to fix this was to manually slide the valve open and close a few times to free up the obstruction causing it to stay closed. At one point it was attempted to use the float valve set up to add calcium hydroxide solution, but this failed within just over a week. Reason for the failure is that the water outlet within the float valve is fully submerged in the sump water. When the saturated calcium hydroxide solution gets into contact with the saltwater, which has a different pH, then solution is super-saturated and therefore some calcium carbonate precipitates. This precipitation caused the float valve to freeze in the closed position and stop working totally. However, these float valves will work for a significantly longer time before they need to be cleaned if the outlet is above the water level. But they will still eventually need to be removed and the calcium carbonate deposit removed.