I just got the funds together to make a decent home for my Betta, a 5 gallon tank (he was in a 1/4 gallon box) with filter and heater. I cleaned everything and set it up. It hasn't really had time to cycle but seeing as it's going to get down to 4 degrees where am at tonight I decided it was best for him not to freeze to death and for me just to keep a really close eye on the ammonium levels. I've tested twice in the last six hours and his levels were perfect 0.0 ppm, I'm using the API water test kit so I'm fairly sure the results are accurate and I'm a Biologist so I'm certain the test were administered correctly.. His water temp was raised over 6 hours to prevent shock and he is currently at the correct temperature (73 degrees). He ate a few hours ago but now he's just hanging near the surface his fins like a little clumpy and he's pale.
What could be wrong and how can I fix it?