I have 2 of those small preset (78 degree) heaters, I put them both in my 20 gallon long assuming they with both heaters they might get the heat up to the mid 70's, however after 3 hours they are only getting the temp up to 73? I dont like adjustable heaters...will these heater eventually do the job and get the temp up? Is it just taking a while because of the gallonage? Or will I need to get a different heater?
What wattage of heater and how cold is the room? Preset heaters that are working will stay on when the tank is cooler than their setting, but having them both on may not be enough if your room is cold. A typical recommendation is 100 W to raise 10 gallons 10 degrees. If the tank isn't to the set point by morning, I'd assume they are either too small or not heating.
never never never never never buy preset heaters.....
ok enough of that..
what i have found is that very often preset heaters don't reach the stated temps...they tend to stay on the low side...that way folks can't try to sue them because their product fried their fish....but since the heaters were only in for 3 hours you need to give them more time to reach their setting.....
i try to stay away from heaters under 100 watts..100s are for my 10s and 15s...200s for 20s and 30s..300s for anything over 30..but that me ; i like the higher wattage heaters because they don't have to work as hard to maintain temps where i need them to be..i keep many of my tanks in the low-mid 80s.
give it time it may get there however preset heaters take away one of your biggest advantages when combating potential fish disease not to mention most tropical fish like the temp higher than a preset can keep it. that said I have a preset in my 10g with the mystery fish and it keeps the water at a steady 78 however the room it is in is about 73-76 at all times
55g#2- 2 discus, 1 angle, and a clown loach
30g- Black ghost knife
20g long-electric blue crayfish
10g-mystery dwarf cichlid