Overheating makes sense, since August is often the hottest month.
Either get rid of the tank or lay it on it's side and leave it open. Or add fans to draw off some of the heat from the lamps. When you say two T8 lights do you mean two tubes or two fixtures (which can have one or two tubes). Two tubes are probably not enough when the plants are growing. You need at least 4 tubes.
Using Barry Rice's light calculator you can get a ball park reading if you have enough light.
Some of Barry Rice's tips on terrarium building. You can get some useful ideas from there.