I have a 50w sealth heater in my 5 gallon ADA with a coralife digital thermometer, and I've been noticing the the thermometer keeps going up and down from 78 to 82 way too often. I have the heater set at 78 and my room temp appears to be around 72, yet I can't keep the tank temp to remain at a constant temp. Could my heater be faulty which is making it go and off so often or are the coralife thermometers pretty untrustworthy? Also I was thinking about switching to a 25w heater, in case the 50w is too much.

i have the same thermometer and it seems pretty trustworthy. the temps fluctuate a little but usually stay right around my set point of 78. I would check the heater

I tried 3 different heaters in my biocube and the temp was always all over the place, then i got a RKL and now my temp stays between 80.2 and 80.5. best hundred bucks ever spent.

I was about to buy another heater, but it looks like I'm better off spending an extra $70 on the RKL.