we have a 4 foot tank approx 200L, we have problems maintaining the temperature we have bought 4 heaters different brands all 300w, set on 28oc, turns off at 23oc,we have two thermostats both read the same cant figure out whats going on , surely we cant have 4 faulty heaters, turned temp up to 30, but no diff still only gets to 23, any ideas HELP