I have one 20 gallon tank in my living room that is consistently unsuccessful. It has a heater set to 78 degrees, a sponge filter and hang on back with sponge instead of carbon filter. This tank has been set up for a year with consistent normal parameters. My other tanks in my bedroom have not had the same problems.
Here’s what I mean by unsuccessful. I have had white clouds, cpds, bettas, platys, snails, plants including moss, octopus and crypts in the tank and rotated as I noticed them not thriving. The fish spent most of the time hiding and while they ate, I noticed they were getting skinny. I had groups of 8 (except the betta). This was particularly obvious with the cpds. The snails would die off within a few weeks of putting them in. The plants would be growing and green for about 2 months then suddenly die back completely or to less than half the leaves they had. I haven’t had an algae problem at all in this tank.
I have done up to 50% weekly water changes when I noticed a problem. The fish were quarantined prior to going into the tank and again before moving to a new tank when I noticed they were struggling. Plants have gotten easy green and root tabs with no improvement.
At this point I feel like I should completely tear down the tank, clean everything and restart.
Does anyone have any ideas what could be the cause? Better recommendations on what to do other than tear down?