Well Karen, you know about our AC problems - we have bills like yours just keeping it on 78, which is why we are going to bite the bullet and have both our AC systems replaced next week.
Anyway, according to HL&P/Reliant/Centerpoint - whatever their name is this week - for every 1 degree you lower your thermostat below 78 it increases your cooling costs 9%.
You say your AC is running constantly - I don't know how big your system is, but I know our 6 ton system (2 units) pulls around 35-40 amps when it's running. If your AC is running several hours extra per day to keep it at 74 for your fish tank, that's a lot of waste. OTOH, my 1/4 HP chiller draws 5.6 amps and runs a total of MAYBE 2 hours a day. To put that in perspective - running my chiller 2 hours a day uses as much energy as running my AC for 15 minutes. There's no comparison. Buy the chiller and raise your thermostat to 76-78 and the chiller will pay for itself in a year or two.
Another reason I like having a chiller - insurance. AC systems have a nasty habit of failing, usually in the hottest part of the summer. Last summer, ours went out twice, and each time it was out for 3-4 days (lousy home warranty company - but that's another rant :-x ). The temperature inside the house was in the low 90's when our AC was out - if I had my tank setup back then, it would have been toast without a chiller.
