My oscar got hurt a while ago and I had to seperate him for a while. He got ick, so I treated that with quick cure. He then got body/fin fungus and I treated with salt. They say use 1 tablspoon for every 5 gallons but I halfed the dose cuz it seemed like too much salt. Ammonia causes their gills not to function very well. They drown.
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How is he doing? I though my oscar was going to die cuz he didn't do much in that tiny 10g I had to put him in. He just sat on the ground up until the last day I had him in there. I think he was just resting. If what you added was bacteria and not something that just make ammonia disappear then it MIGHT have caused your level to truely drop that much. If you're using the liqiud testers, make sure you get all of the rinse water off using tank water so there isn't a chance of a foreign substance (tap water) in your test. If it's the strips, they're not always 100% accurate. I control my ph by controlling my hardness by adding good bottled water in place of some of the tap water. My Tap water hardness is really high, and botled water has no hardness hardly, so I put 50/50 tap/bottled in my tanks. That keeps my ph at 7.
Footnote: it's actually pH not ph.
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Forgot to mention, salt helps their gills function better.