Automatic top off
Based on evaporation, the water level? Based on Salinity? Based on Electronic conductivity?
Based on a float switch, the water level, seems to me not to be a good idea. What if there is a small leak.?
EC of seawater is high, I think this makes it difficult to make it accurate enough
for a reasonable price, and without constant maintenance of the probes.
By using LED, evaporation is greatly reduced. Covering the tank, the sump or and other refugia where possible will limit evaporation.
Through experience one knows approximately how much water evaporates every day. One can use an adjustable drip tap, such as a baxter in the hospital (ask a nurse) to drip the water (also for lime water). What is available is used to check the salinity. In this way, overdosing in the event of a small leak, even after a few days of absence, is prevented. The advantage is that it is very cheap.