REALLY mad right now!
My 20g was a starter kit. It came with a 100W heater with no brand name written on it, that had no numbers to set the temp by. I didn't think much of it and thought I'd replace it at the first opportunity. It has been happily keeping a steady 78-79 F in that tank for months now. It will stay.
In my 3g I originally put a Hydor Mini Theo 25W, and my 5g had an Elite 25W. The Elite is not fully submersible and therefore not a favourite, and I blamed this for it not (seemingly) keeping a consistant temperature in the 5g. Meanwhile I was thinking about putting the Hydor in the 5g and getting a Marineland 10W preset to 78 F for the 3g (it was rated for that size). I ordered the 10W from Big Al's. It did NOT keep a consistant temperature. The temp fluctuated by as much as 10 F in a 24 h period. This is when I had oto's in the 3g and I lost one of them at that time, due to the temp or not I don't know (I had just got them). I complained to Big Al's, they told me to send it back and IF they determined it to be defective they would reimburse me for the heater plus shipping. That is what happened.
So the Hydor went back into the 3g and that worked well (then anyway). Meanwhile I found the 5g always seemed to be too cool, so I ordered (from Big Al's) a Hydor 50W. I then discovered that the temp problems in that tank were due to a drafty window! fixed that. Still didn't like the Elite semi-submersible, so I bought a Marineland Visi-Therm 25W from the store location in Calgary of Big Al's (store locations and online store are, apparently, different companies they tell me). Got the heater home, opened it up...no suction cups to attach it to the aquarium! Drive all the way back there (40 min, this is why I usually order online) and they gave me suction cups (didn't exchange the heater). The Visi-Therm was installed in the 5g yesterday, and is now making it too HOT. If I set it on 73, it keeps it at an even 79 !!! Meanwhile the Hydor 25W is having the same issue in the 3g - I need to set it at about 72 to keep from boiling my
betta !
The heaters in the 3g and 5g I don't personally feel are defective. They are in small volumes of water which will promote temperature issues to begin with, and then my house has thin walls and drafty windows and is prone to large temperature variations, especially since the weather's been a little wild here recently - 40 degree temperature swings in a 3 day period. Both these tanks have high-powered fluorescent lights which are likely at least partly to blame for the temp increase throughout the day (as much as 4 degrees). I've tried several different thermometers so I know that's not it. But how to keep a constant temp in my little tanks? I don't really know..
So I still had the 50W Hydor I'd bought for the 5g, now replaced by the Visi-Therm. That was one of the reasons I bought a 10g tank

Set that up yesterday - when I filled it I used lukewarm water (around 80 going in) and installed the Hydor.
This morning the temp in the tank is 74, and the Hydor got re-set to 84 - two hours later, still 74 !!! So I called Big Al's online - they told me that since it had been used, I could send it back and under warranty they would replace it with another of the same IF they agreed it was defective. I would be responsible for the shipping in this case (??!) and if they did not determine it was defective, they would return the same item back to me and I would be responsible for the shipping costs for that too. Based on returning the 10W to them, shipping one-way is $15.
I am irate with Big Al's to say the least. I just ordered some air pumps from them yesterday when they were out of stock in the store. I'm going to monitor the new tank for a bit and possibly move equipment around between tanks, and it seems that now after paying $32 for a 50W heater for the 10g I will still likely have to add a 25W as well, either the Elite or yet another new one.
Sorry just had to rant.