I needed a cheapo heater for my RO/DI water also, When i was at SB, Tony had a used 100 watt he sold me for $5. Don't know if he has more. Give him a shout. A 300 watt does sound like alot. I use 2 200's to heat my whole tank (90 + sump= 107 gal, less the displacemnt for the rock and sand), and they do the job just fine.