So had the water tested - levels are all fine. No ammonia or Nitrate issues. Gave him the pea treatment again tonight. He devoured it like he was starving. He's still feisty, lively, and swimming - it's just that his back end sinks. He also seems to lie on the bottom on his side. I emptied half of the water so he has no trouble coming up for air and I can do water changes instead of using the filter. Can he live happily like this?