Loaches Online Forum

Few months ago I have a problem with nitrate. After I switch to RO water and fill the 75 gallons sump with Hornwort and Anacharis plants. My nitrate been 0 for a month now. But my water is evaporate so fast i have to fill 50 gallons of RO water every week. Is that mean i dont need to do water change?

Unfortunately, the fish in your tank will also make other stuff which can harm them.

Therefore you will still have to change water. Is there anything you could do agains the evaporation? If you could, you could just go on, but now drain 50 gallons of old water every week.

As far as I know, I'm not a sump user, a sump is intended to have a closed top

Nitrate is the most common parameter by which we used to judge about water quality long term. But it's not the only thing that accumulates and is harmful to fish.
Get yourself a TDS meter. They are cheap enough now and last for ages as they are electronic devices. When water changes are not done often enough, the reading increases over time away from your tap water stats, thus the water quality goes bad, regardless of your nitrate levels. I have tanks which have very low nitrates consistently due to high plant mass and I still do weekly water changes. The TDS reading could double or treble over time without water changes. Fish aren't happy.

There's one thing more. We have to remember about the water hardness in the situation like that. When water evaporates - that's clear H2O. All salts, which increase the hardness still remain in the tank. We have to remember about that. Of course when we refill evaporated water with 100% RO water that's ok. But if even small part of added water has some hardness - it will increase the hardness in the tank. And after some time of repeating that scheme the increase of water hardness could be so huge.

I didn't add any minerals on RO water. It's pure Ro water with 6.4 ph. When i first start this hobby i didn't know what i was doing. I keep killing fish with tap water, so i use Ro water. i kept the arowana with pure ro water with no water change for 2 years. I have a friend also keep fish with pure ro water no minerals added. My LFS also use ro water on his fish, he told me he didnt add any minerals to it either.

My fish is very happy in it. I can see the different. I just never thought that plants suck up so much nitrate (i always overfeeding). My anacharis and hornwort are growing fast.

There are probably a very small amount of minerals in the water. Even RO has some stuff. Not much, though, unless the membrane is getting old.

If there is debris on the bottom of the tank or sump, I would do just enough of a water change to clear out most of the debris. Perhaps once a week, as little as 10% might be enough, with a large tank.

The plants are removing most of the waste from the fish food.

Still, it is a good idea to get a TDS meter. 2 reasons: You can test the RO water, and know when it needs a new membrane. Also, you can monitor the accumulating minerals and salts in the tank, so you can plan water changes.