Salt doesn't evaporate. Water does. Every litre that leaves the tank as vapour makes what's left saltier, and in a Brisbane January this tank loses four to five litres a day. For two years I dealt with that by carrying a jug from the laundry every evening, and my salinity chart looked like a mountain range.
I bought a refractometer in 2023 and started recording salinity daily for a fortnight, just to see. The tank was nominally at 1.025.
| Day | Morning | Evening, before top-off | Litres added |
|---|---|---|---|
| Mon | 1.025 | 1.028 | 4.5 |
| Tue | 1.025 | 1.028 | 4.5 |
| Wed | 1.025 | 1.029 | 5.0 |
| Thu | 1.026 | 1.029 | 4.0 |
| Fri | 1.026 | 1.030 | 5.5 |
| Sat | 1.027 | 1.031 | 0 — away overnight |
| Sun | 1.031 | 1.027 | 9.0, all at once |
Look at Saturday and Sunday. One night away and the tank climbed to 1.031, then I dumped nine litres of fresh water in and dropped it four points in an afternoon. That's a bigger swing than anything that happens on a natural reef in a year, and I was doing it to my corals most weekends.
The corals never died. They just never really grew either, and I spent eighteen months blaming my lighting.
Nothing clever, and nothing expensive. The parts list:
The important bit is the two float switches, wired in series. The lower switch is the working one: when the sump level drops, it closes and the pump runs. The upper switch is the safety: it sits about fifteen millimetres above the working switch and cuts power if the level ever gets there. A single float switch is one stuck magnet away from emptying 25 litres of fresh water into your reef, and I've read enough accounts of that happening to want the second one.
Salinity now sits between 1.0249 and 1.0252, which for a refractometer read by a person is the same as saying "it doesn't move". The chart is a flat line.
The effects I did not expect:
I plumbed the first version with flexible silicone tubing and it kinked behind the cabinet where I couldn't see it. The pump ran, nothing came out, and the tank climbed for three days before I noticed. Rigid airline with proper clips, routed where you can see the whole run, solves that permanently.
I also, once, filled the reservoir from the wrong container and put about two litres of saltwater into it. Salinity went up rather than down, which is exactly the opposite of what a top-off is for. The reservoir is now labelled in black marker on two sides, and I only ever fill it from the RO drums, never from the mixing bin.
You can get most of the benefit for nothing by doing two things. Mark your sump's correct water line with a strip of tape, and top off small amounts twice a day rather than one big pour at night. That alone cut my daily swing roughly in half back when I was still doing it by hand.
And measure with a refractometer, not a swing-arm hydrometer. I had a hydrometer for the first year and it read consistently two points low, which meant the tank I thought was at 1.025 was sitting at 1.027 permanently. Calibrate the refractometer with proper calibration fluid too — RO water is better than nothing but it only calibrates one end of the scale.
Next one to read, if you are setting a tank up from scratch, is the cycling log. If you just want to know what is living in here, that is the livestock notes. Questions go in the note box on the about page.
Glass Reef — a hobby diary typed up in a spare room in Brisbane, Queensland. Nobody sponsors it, there are no affiliate links anywhere in it, and there is nothing on it for sale.
Copyright © 2019–2026 Dale Prentice. Back to the front page.
Last fiddled with on 18 October 2025, when I added the bit about calibration fluid.