I am a new fish owner with 4 tetras and 1 pleco. My tank is 10 gallon (37.8 liters) with an Aqueon QuietFlow LED 20 PRO Aquarium Power Filter
The tank has been running for over 3 months and it gets a 40% water change (approximately) every three weeks with distilled water.
Tetra EasyStrips 6-in-1 Aquarium Test Strips for Fresh/Salt Water give these values:
How can I bring this tank into balance?
Firstly, I'll go for my old refrain "consistency is key". It's better to have consistent water parameters than changing them continually to 'chase the perfect conditions'.
Firstly, your nitrite should be 0 if your tank is correctly cycled. Secondly, you haven't got a reading for ammonia there (which is one of the problems with the test strips). Ammonia, as I'm sure you know, is lethal to fish even in small amounts. Check your ammonia reading and ensure it's also 0 which should allow you to see if your tank is cycled. In all honesty, after 3 months it should be cycled but you can trigger 'mini-cycles' if you change all of your filter material or rinse it in tap water etc.
The nitrate is not bad and you can reduce that figure with more regular water changes. There is a lot of discussion on the perfect amount of water changes. Personally, I do 25% every week (every week against all odds) but so many people have so many different views, I'll leave you to read all of those.
If you're changing your water with pure distilled water you'll want to be careful. Distilled water has no minerals in it (like reverse osmosis (RO) or RO/DI). You need to re-add the minerals to it, or your pH could crash. If there are no buffers in the water, the pH can swing wildly and kill the fish. Remember that pH is a logarithmic scale, so even the smallest change can have a big effect. Here is a great answer about RO and distilled water.
