2 Different Heaters Wont Heat My Tank

Okay, so I've got a 20 gallon tank. It has 3 corydoras in it. Its planted with eco complete substrate, 1 piece of driftwood and a couple rocks.
I am having an issue with the heater. I had a 75 watt heater that just wasn't cutting it. At first I thought it was the thermometer so I went out and bought a different kind - one that suctions to the inside of the tank. It read the same - somewhere under 70 degrees Fahrenheit. So I went and purchased a 100 watt heater. Still the thermometer reads the same. I tested the thermometer and it works just fine, so what gives? I want to add more fish, but until I can get this figured out, i'm stuck.

EDIT:
the 75 watt heater was adjustable and I had it well up into the 80's at some points and still no change in temp. I've also tested the outlets and the such, they all work, too.

 

Is the tank covered?

 

What's the temperature in the room. I'm first curious if you're having this problem with an ambient temperature of 50 or 70. If the room is 70 and the tank is 70 even though the 100w is set to 80 for days? Or is this happening with a house that's 50 degrees and a heater set to 80 but still only reaching 70?

 

Is the tank covered?

It is, yes. It has a hood with an LED light strip. The light is on a timer. On from 8 am to 8:30 pm. Theres a slight gap in the back where the filter sits. It was the 2o gallon kit that petco sells. Made by Aqueon. I'm replacing the filter (new one will be here Friday).

 

Marineland has a "heater selection guide" based on the heaters ability to change the temperature up to 15 degrees difference. Their 100w heater can heat a 29 Gallon tank to 76* as long as your house is warmer than 61*F. That could be a factor depending on where u live this time of year

 

What's the temperature in the room. I'm first curious if you're having this problem with an ambient temperature of 50 or 70. If the room is 70 and the tank is 70 even though the 100w is set to 80 for days? Or is this happening with a house that's 50 degrees and a heater set to 80 but still only reaching 70?

the temp in my apartment is 75. It stays at 75 and the heat kicks in if it gets lower. I've had the 100w heater in for about 6 days. The 100 watt heater is NOT adjustable, so I'm not entirely sure what temp its putting out. the 75 watt one was, though. it was at 80 for about a week while the apartment was at 75. I'm happy to get an adjustable heater of a higher wattage. basically whatever I have to do to keep my fish from dying and to quit wasting money on things that aren't effective.

Marineland has a "heater selection guide" based on the heaters ability to change the temperature up to 15 degrees difference. Their 100w heater can heat a 29 Gallon tank to 76* as long as your house is warmer than 61*F. That could be a factor depending on where u live this time of year

I was thinking the same. It gets pretty cold here around this time. Right now its 59F, but its been as low as I wanna say 11F. The tank is somewhat nearish a window, too.

 

That's very frustrating that the tank is dropping below room temperature. There are some good articles with heater ratings that describe reliability. I try to get the highest rated bc a bad heater can cook or freeze the fish

 

I’d definitely get an adjustable heater. The pre-set ones aren’t nearly as reliable.

 

the temp in my apartment is 75. It stays at 75

Right now its 59F

Oh good. Maybe just higher watts on reliable heaters. Try using a 100 and a 75 together.

 

I’ve had a problem like this, but it turned out that the outlet I was plugging my heaters into was just broken or disconnected or something.

 

That's very frustrating that the tank is dropping below room temperature. There are some good articles with heater ratings that describe reliability. I try to get the highest rated bc a bad heater can cook or freeze the fish

Any suggestions? I've made a few decisions on my own and now I'm a little hesitant to trust my own judgement!

I’ve had a problem like this, but it turned out that the outlet I was plugging my heaters into was just broken or disconnected or something.

I thought so too at first! but I plugged in my phone charger and it works just fine. I was pretty much pulling my hair out over this!

 

Try plugging the heaters into a different outlet just to see if they get warm. If they don’t, then you must have really bad luck with heaters

Try plugging the heaters into a different outlet just to see if they get warm. If they don’t, then you must have really bad luck with heaters

I call some of the outlets at my house “picky” since they only work for some things. (I must have had a really bad electrician who did my wiring)

 

Are you putting the heater in the subtsrate? I did that and had the same issue and could never figure it out until I pulled it up to clean one day didn't set it back and it has worked great ever since. I know heaters also need to be near the filter intake as it helps spread the heat throughout the tank. As for Corys my GF is obsessed so I've spent a long time researching them and have found out it's best to keep them with the same species of Cory and that usually they require lower water temps for tropical fish peppered Cory's are the most sensitive to heat from what I've experienced and also require at least 6 or more of them in a school. We've tried having all different kinds and it never worked. My parents have bronze and emerald Corys together so that's not always the case. Just depends what breed we have some sterbaI and they are awesome to watch!. Just make sure whatever fish you add don't crowd them out of the bottom space and don't overstock!!! When I first started out that was my main issue

 

What kind of heater do you have? I really like Eheim Jaeger, I have a 75 w for a 20 gal and the room can get fairly cold (low 60ies). The heater has no problems heating my tank to 77 degrees.

Is the heater close to the filter? I always put my heater next to the intake so that the warm water gets distributed.