Stabilizing 30L tank

Gypsum

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I have an established 30L nano planted tank which seems to be experiencing large PH swings. The tank started life as a fry/grow-out tank for corys, but the juvenile corys moved in with their parents in February. It currently contains three least killifish (Heterandria formosa), out of a population that was once five, a couple assassin snails, and some Malaysian trumpet snails which hitchhiked on a plant. Its plant population is anacharis and a hygrophilia, and it has a small piece of spiderwood.

I started tracking PH after we lost two of the killifish. They just disappeared. Can't find them. Anyway, immediately after a water change, it was 6.5/6.8-ish, which is roughly what comes out of the tap. Ammonia, nitrites, and nitrates were all 0. Then two days later, it was around 7.6 (the colour in the test water was very blue). Not ideal. And ammonia was .25 and nitrate at 5ppm. I know it's normal for people to have nitrates, but those always test at 0 ppm on our other (bigger) tanks, and 0 on this one a couple days ago. We obviously did a water change.

I know tiny tanks are tricksy to maintain because they are very unstable and do things like this. This one gets 50-70% water changes twice per week (whereas our assorted larger tanks get fresh water once per week). How else do people stabilize their nano tanks?
 

Guyb93

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Yeah my question was about substrate? Are you using crush coral sand ?
would that be enough of a ph swing to kill the fish ? Being a gradual change
 

Guyb93

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It has sand. Probably about an inch deep.
Some sands can alter the ph of the tank such as crushed coral or sea sand , I’d take a table spoon of your sand put in a glass of water and test see if it changes
 
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Gypsum

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It isn't crushed coral or sea sand. We've used the same bags of sand in our other tanks, including one with apistogrammas and otos that's only 70L.
 
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Discovered that the in-built filter was not properly attached. Hasn't been since we bought the tank. Whoops. Tank testing at 6.8 today, three days after a water change.
 

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