I'm getting ready to set up my 100G and I know there is a formula for watt to gallon size. But I cat seem to find it. So can anyone tell me how big of a heater I need for my 100g??????

I don't think you will need any more than 400 W for that tank.

Or 2 200 W incase one would fail you would always have a back up. But OFL is correct 400W would be perfect!

Codeman_T USMC wrote:


I am not disagreeing with you but the way I look at it is you can easily keep your water at around 28°C (82°F) if you have a 400 W heater. These heaters are really inexpensive nowadays so even if one did pack up, you can order a new one and it would be with you in a day or two. Even then your water would not be too cold if your house is still warm. It would save all that hassle of putting all those heaters inside the tank.

Or better still, just have another heater on standby

Thanx for the replies. I'm gonna go with a 400W, now I just have to find one...........

What chance would a 300W in a 90 gal (US gal) tank have of keeping the water at proper temp? I was looking at a 300W Rena heater that can hookup to the Rena Filstar I am getting...that was I won't have a heater showing at all in the tank....would this possibly work?

I use that Hearer in my 125, and it works GREAT!And the Oscars can't break it!

Yeah that is why I wanted to use that one...heard the oscars can be buttheads and break heaters sometimes...plus i'd rather not see it if I don't have to.

Glad you use the same one and it works good on your 125...should be good to go in the 90 I have then! Thanks for the input!

300W heater is good enough for a 90 gal tank..Rena is a good and durable brand.

Awesome...that makes up my mind on that part of the setup at least! I'll have to go pick one up this weekend.

some people pick smaller and cheaper unit but later complain that it doesn't work.
the truth is it works but doesn't work well because it is under capacity. heater
and filter is one investment needed in this addicting hobby..heater is optional
in a place like mine..... normal temp on ordinary day is 27 deg C

I'm gonna research the rena 300W some............