It's very hard to give informed suggestions without a photo, too. However, a low temperature of 75 will cause lethargy; also, if your Nitrite is .5 *after* a 50% water change it was high enough to compromise your boy's immune system. I would suggest three things:
1. Get a liquid Ammonia test. The Ammonia level is the most important parameter for an aquarist to know. Liquid tests are more cost efficient than strips and you'll be doing a lot of testing for Ammonia.
2. Get a bottle of Seachem Prime. It is dosed at two drops per gallon.
3. Follow this tutorial and cycle your tank. It also explains the reason for the Ammonia Test and Prime. http://www.bettafish.com/30-betta-fish-care/507585-cycling-two-sentence-tutorial.html
When cycling a tank I do 25% water changes if Ammonia or Nitrite reach .25ppm; but .50ppm is okay. Once your tank is stable with 0ppm Ammonia and Nitrite and Nitrates under 15-20 you will only have to do one 25% water change per week at your current stocking. If you add more you will have to keep track of the Ammonia level for a week or two to make sure everything stays stable.
1 - 2 of 2 Posts
This is an older thread, you may not receive a response, and could be reviving an old thread. Please consider creating a new thread.