I have a 10 gallon tank with the same gravel (it's too big, an early mistake, but I'm just letting it go until I'm ready to do soil&sand), similar Hang-On-The-Back (HOB) filter.
I would bring up the water level to a level so the filter doesn't cascade any more, and clean or replace the bulbs with probably 8 watt Compact Fluorescent Light (CFL) bulbs (twirly bulb) and clean that great reflector you've got (I think I saw one in your pic.) And you need more plants, like a good bunch of planted Water Sprite (the kind with roots to be planted) which will grow well. For now, don't get the floating kind which would cut down on the light on the rest of the plants below.
Idk about your fish. I wouldn't add any more fish until your tank has more plants in it.