it's "too much" when you trip a breaker because of the draw.
Is the AC on the same circuit as the tank? (Is it a window or wall unit in your son's room, or do you have CAC?)
As far as the stuff related to the tank goes, you should start by adding up the ratings of each of the components (lights, pumps, etc). Lights might be .5A, pump #1 2A, pump #2 3A, heater 5A, etc. Maybe a lot more, maybe less.
Take that total, and multiply by 1.25. That number is the rating of the circuit you'd want (dedicated) for the tank. I'm assuming that most of the load would be continuous. Maybe it wouldn't be (the heater probably cycles on for < 3 hrs. at a clip), but it couldn't hurt to have some overhead anyway.
If the number you got in adding everything up * 1.25 is low (5-10A) you don't generally need to bother with a dedicated circuit, though it would still be a good idea if your son's room is sharing a circuit with other rooms. (If your son has a TV/stereo/computer in his room, that adds to the load, assuming that right now his room only has 1 circuit powering its receptacles. If there is an AC in his room running off the same circuit as the tank and/or other appliances, then the AC should also have a dedicated circuit).
As Scuba_Dave pointed out, you can always distribute the load of the tank equipment across more than one circuit, which will help keep the fish alive in case one circuit blows, and keep the load balanced.
We sometimes have issues with running the microwave in the kitchen and hair dryer in the bathroom at the same time and circuits tripping.
You might want to consider having an electrician take a look. I only say this because it sounds like you have too few or strangely laid out circuits or a lot of heat in your panel, and it could take us a while to sort it out here.. But you could post some more info; panel mfr./model, age of your electrical system, service rating (50A, 100A, 200A, etc).
BTW - I'm not an electrician, just putting in my 2c.