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I don't have a master test kit, so I can't use that.

How often should I change the water, and what should the percentage of water changed be, in a 90 gallon with:
- 5 inch female RES
- almost 4 inch unsexed Midland Painted Turtle
- seven 1 inch giant danios
- two to ten 1 inch rosy red minnows
- two Aquaclear 110 HOBs
?

Thanks in advance

I do 50 percent once a week. I think that's a good figure but your ammonia might still be high at the end of the week. But its really not that big of an issue.

Edit- Nitrates not Ammonia. Get them confused sometimes.

Your water tests will tell you how much water to change and how often. There's a ton of posts on this forum on this very topic. You do water changes to keep Nitrates in check.

VeipaCray: I don't have a kit for testing water parameters. I know about the whole ammonia to nitrites to nitrates process (I'm in the process of becoming a family doctor). I was just hoping that a 40% water change every two weeks would be sufficient, as my schedule's very tight.

I do, however, have two AquaClear 110, and while I know that no filter can substitute water changes (the whole closed environment business), keep in mind that I also vacuum their poop daily with a turkey baster, and feed them everything messy (with the exception of vegetables) outside of their tank.

Also, I now have around 23 rosy reds (will soon disappear) and a 1.5 inch comet goldfish (going to see if he lasts).

Turtles are very messy you should have a over powered filter or filters with the right media and bio media all the water levels should be at a reasonable level...I have a 125g tank with 100g of water with a Fulval fx6 for a 400g tank...when I do a water change I do min. Of 50% remove all river rocks and rinse them off