I have a ten gallon set up that i used to keep some tetras an an angel fish in. I used a submersible heater that was labeled 5 - 15 gallons, and it worked fine. I got it back out to set up for my new DT, cleaned it all out (used diluted vinegar and warm water), boiled all the fake plants, bought new gravel and boiled that and rinsed off about a dozen times. I got out the old filter that I had and thought it was in bad shape from storage, so I bought a Wisper internal filter, and quickly felt it was too powerful to leave in while my betta was there.
I set the whole thing up, and added my real plants (which will hopefully start growing soon), and turned the filter on and put the heater in. 2 days later and I can't get the temperature up above 74*. I have two submersible heaters in there now, and am leaving the filter on in hopes of getting it to circulate, but still no avail. I have used these heaters before and they worked fine. They are the kind that only come on if it gets below 78*, but they are hardly ever on, and don't stay on for long. The one is brand new, and the other is 2 years old.
Other methods for keeping temp up or other suggestions for thermometers?
Thanks so much!