Pull the lighting off the APC, its going to do you no good what so ever. 2, 250w halides will suck the power out of that APC pretty darn quick. You'd be better off putting the filters and pumps on the APC. Remember the APC is there to run the tank when the power goes out, the less you have plugged into it, the longer its going to last.
Those small home sized UPS's are not meant to run things very long, thier meant to give you time to properly shut down your PC if the power goes out. They typically only last for about 10 minutes anyways, great for shutting down your pc and not losing data, but pretty useless when power to your tank has been out for 2 hours
Have you test ran it yet? Theres a good chance its old and the battery isn't very good as well. I belive APC recommends the batterys in thier UPS's be replaced every 2 years? not sure its been awhile...