Is your overflow plumbing able to handle full flow from your return pump ?
I ask as I noticed during testing mine that there is a correlation between my sump level and my overflow level. Not just the actual levels, but the water level in the overflow affects drainage speed and so on.
I am wondering;
If your overflow can handle full flow maybe it will help you stabilize the whole setup ?
Good luck

From what I've gathered, you're overflow isn't tuned precisely, and is causing a slow change in the sump level. Albeit a slow change, it is a change nonetheless.
There isn't an ATO on the market that will compensate for a mistuned overflow.

Is your overflow internal? My herbie is internal, and if it were to swing way out of tune and drain the over flow, I'd have an extra 4.5 gallons of water in my sump. That would cause an extreme difference in my salinity also.

I'd make sure your overflow is 100% consistent before you go ahead and drop some money on an ATO that isn't going to help your problem at all.

Have you ever calibrated? If not then its still possible its wrong.

However, depending on your salt, some needs to sit 24hrs to fully dissolve, like H2Ocean. If I use mine right away I can guarantee salinity swings.

Also, do you turn your ATO off before water change? Maybe its pumping extra top off in while you remove dirty water. Just a thought. Maybe you are not adding EXACT amounts of new water to removed water and then ATO is diluting to bring top up to correct water level?

Just think the solution is a simple fix.

Good luck.



Just makes no sense at all that the ATO is doing it.

Just needed a little more dialing in guys. Seems to be holding now.

Thanks for all the insight.