Help? Inactive Clownfish

Hi. First of all, I'm new to the site, and the hobby, so I apologize if I sound kind of dumb. I know this may seem like a lot more information than necessary but I honestly have no idea if any of it could be related to my problem. Anyways, I just started taking care of my family's 14 gallon Oceanic Biocube. The tank has been up for about 7 years now, but has unfortunately been through a lot of neglect in that time. For the first two years it was running pretty fine. At first we had a true percula clownfish and a purple firefish with a cleaner shrimp, emerald and hermit crabs, snails, and a few corals. Everything was fine until 10 year old me convinced my dad to put a six-line wrasse, an archeye hawkfish, and a bubble tip anemone in the tank. Within a week the hawkfish had killed the cleaner shrimp, the wrasse, and the dartfish, and as if things weren't bad enough at that point, the anemone split in two a few weeks later and died, followed by a massive outbreak of cyano bacteria. At this point my dad was becoming less and less dedicated to general maintainence and the tank continued to fall apart. After a few months almost all of my clean up crew and coral had died off. The hawkfish disappeared about a year later, leaving only my percula clownfish. Fast forward to now and I have gotten the tank back on track. The water parameters are all fine (with the exception of temperature which fluctuates between 69 and 73 degrees between night and day. I am saving up for a heater to correct this soon), the cyano is almost completely eradicated, and a few of my corals have even made a miraculous comeback! Now here's my problem; the clownfish won't leave the rocks. Up until about a year ago the clownfish would swim around the tank and would always get very excited during feeding time, rushing to the surface to eat whatever it could grab. Now it just swims back and fourth behind the rocks, poking its head out of a few different holes and only eating whatever food flies directly in front of his face. This has me concerned because it's leaving a lot of uneaten food around the tank. The only time he ever comes to the surface is when the lights turn off for the night so he can sleep on the grate of the overflow. He is still the only fish in the tank and the only new live stock I have added is a small cleanup crew of snails and crabs. If anyone knows why he is doing this and/or how I can get him to swim around more and come the surface for food I would be greatly appreciative! Thanks :)