Your best bet is to check everyday for a while to see when your temps rise to a certain level with the 400 watt halide on, then put the ballast on a timer with a shorter photo period (ie, run your 250 watts from 11-9 and run your 400 watt from 11-6 or however long it takes to overheat your system). You may also consider putting in a lesser watt bulb as well if heat remains a problem. Some ballasts allow you to switch to lower watts without changing the wiring etc. I like using 175 watt and 250 watt halides to avoid using chillers or over heating your system. Plus, I am responsible for the electricity bill, so less is more for me .