Winter Evaporation Is Draining Your ATO Reservoir Faster Than You Think
Posted by Gallery Aquatica on
Winter throws up a subtle salinity problem. You test and get a reading a touch higher than usual, say 0.002 above your normal 1.026, even though nothing has changed in your dosing or feeding. The usual culprit is the ATO reservoir running low while the float switch quietly fails to trigger a full cycle for days. It is not dramatic, just a slow drift that catches people out because nobody is watching the reservoir closely in winter.
Dated Alert: 24 July 2026
Brisbane conditions this week: cool, dry overnight air combined with heaters cycling more often is pulling extra water out of the tank through evaporation, faster than most ATO reservoirs are sized to handle without a top up. Salinity creep is easy to miss until it shows up as coral stress or fish behaviour changes.
Why This Matters More Right Now
Evaporated water leaves salt and minerals behind. Only the water disappears, not the dissolved solids, so every litre lost without replacement pushes specific gravity up. In a display running on autopilot, a reservoir that lasted a comfortable week in autumn might now be dry in four or five days. If the ATO can't draw water because the reservoir is empty, the pump keeps trying, the float sits low, and salinity keeps rising unchecked overnight while everyone is asleep.
What To Check This Week
- Top off your ATO reservoir today and mark the waterline with tape so you can see the draw rate over 48 hours.
- Test salinity with a refractometer rather than relying on a hydrometer, cheap hydrometers drift and won't catch small creep. Browse refractometers if yours is due for calibration.
- If you are topping off manually, increase frequency to daily rather than every second day until the cooler nights ease off.
- Check your RODI unit is keeping up with demand, a slow membrane means you are stretching stored water further than it should go. See options under ATO systems if you are still topping off by hand.
- Inspect heater placement and wattage under heating and cooling, an oversized heater cycling constantly accelerates evaporation beyond what the reservoir was sized for.
None of this needs a big fix. It needs five minutes with a refractometer and a habit of glancing at the reservoir before bed, at least until the pattern of cool nights settles down. Products and gear mentioned here are drawn from our regular maintenance range if you need to top up supplies before the weekend.