80 watts is barely enough for a 75 gallon tank, and it will not be enough for any but low light plants. That may be your problem for now. 160 watts would be enough, but I doubt that you have reflectors for those bulbs, so that isn't anything like an AH Supply light kit with excellent reflectors. My Excel bottle tells me to dose a capful of Excel daily in a 45 gallon tank, so that would say you need about 2 capfuls a day for a 75 gallon tank. This will give you a slow growth, relatively low light tank. If you want to make the plants grow faster, and grow more demanding plants, I suggest buying an AH Supply light kit, at least a 96 watt kit, for example. Then, if you use presssurized CO2 you will be up to speed.