Evaporation raises salinity in every saltwater tank. An auto top-off (ATO) replaces that lost water for you. It keeps salinity stable and reduces daily stress.
What an ATO does, and why it matters
An ATO adds fresh RO/DI water as the water level drops. It does not add saltwater. Salt stays behind when water evaporates. That is why salinity creeps up without top-off.
Stable salinity supports fish breathing and coral tissue health. Aim for 1.025 specific gravity, or 35 ppt. Try to keep daily swing under 0.001 SG. In small tanks, swings happen fast.
Most ATO systems use a sensor and a small pump. The sensor detects low water level in a sump or rear chamber. The controller runs the pump until the level returns. Many units add a safety timer.
ATO also protects equipment. Return pumps hate running dry. Skimmers overflow when sump level changes. With steady water height, skimmers stay tuned and quiet.
Before you buy, estimate evaporation. Many tanks lose 0.5–2.0% volume per day. A 40-gallon system may lose 0.2–0.8 gallons daily. ATO turns that into tiny, frequent additions.
- Top off with RO/DI water at 0 TDS when possible.
- Calibrate salinity with a refractometer and 35 ppt fluid.
- Track evaporation for one week before sizing a reservoir.
For more on stability basics, see reef tank salinity stability. If you run a sump, review sump setup guide for ideal sensor placement.
Choosing the right ATO and setting it up
Sensor type matters. Optical sensors resist salt creep better than simple floats. Many hobbyists still like dual-float systems for redundancy. The best setup uses a primary sensor plus a high-level backup.
Place the sensor in a stable-water section. In a sump, that is the return chamber. In an AIO tank, use the last rear chamber. Avoid the skimmer section if the level is fixed there.
Size your reservoir for at least three days. For a tank that loses 0.5 gallons daily, use a 2–3 gallon container. Keep it covered to reduce dust. Clean it monthly to prevent biofilm.
Use a small pump or peristaltic pump. Peristaltic pumps reduce siphon risk. If you use a powerhead, add a check valve or route tubing above the waterline. Always test for back-siphon during setup.
Set a run-time limit if your controller allows it. Many hobbyists start with 30–60 seconds max per cycle. Then confirm the system can recover after a normal evaporation drop. This prevents floods if a sensor fails.
- Mount sensors where waves and bubbles are minimal.
- Secure tubing with a clip to stop it from slipping.
- Mark the “full” line on the reservoir for quick checks.
If you also dose kalkwasser, plan carefully. Kalk in the ATO can raise pH fast. Start at 1 teaspoon per gallon of RO/DI, then adjust. Test pH morning and evening for one week.
Troubleshooting and common mistakes
The most common issue is overfilling. Salt creep can jam a float. Snails can also block it. Use a snail guard and wipe sensors weekly with tank water.
Another issue is rapid on-off cycling. This is called “chatter.” It often comes from waves, bubbles, or a sensor set too close to the return pump. Move the sensor to a calmer spot. Add a short delay if available.
Watch for slow salinity drift down. This can happen if you top off too much. Confirm your ATO is not compensating for a leak. Check for wet floors, damp stands, or salt trails. Also verify your ATO adds only fresh water.
Reservoir contamination can cause algae and odor. Keep it dark and sealed. Use food-safe containers only. If you see slime, rinse with vinegar and warm water. Then rinse again with RO/DI.
- Test fail-safes monthly by lifting the sensor by hand.
- Keep a spare pump impeller or backup pump on hand.
- Log salinity twice weekly to spot trends early.
For a broader maintenance routine, read weekly reef tank maintenance. It pairs well with ATO checks.
An ATO is one of the best upgrades for stability. It reduces daily work and protects your gear. Set it up with redundancy, test it often, and your salinity will stay steady.
Sources: Reef Aquarium Volume Three (Delbeek & Sprung); The Reef Aquarium Volume One (Borneman); manufacturer manuals for common ATO controllers and optical sensors.
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