I use simple aquarium heaters with sensors for for my tanks that run from 250 to 300 gallons plus. Biggest heater I use is 1000 watts and it's not on all the time. One of the high schools I set up bought an AES industrial heater and thermostat and it fried in no time. My aquarium heaters have been going strong for 2 years now.
I bought my heaters on Amazon for under $100.00 like this one:
What is your total ammonia, nitrites, and nitrates in your system? I wasn't aware you could measure un-ionized ammmonia with a test strip. Really?
Even below lethal levels of all of the above will stress your fish and produce lousy growth rates. Test strips suck IMHO. You really need to invest in something like this:
