Either the tank gets exposed to temps too cold for the heater to keep up or the heaters are undersized or crap ? I only use Aqueon Pros and have no issue with any of my tanks . I keep several at 86f ! I have over 30 of them . They have lifetime replacement guarantee so I wonder why others buy different heaters. I know many people who have got a free heater from Aqueon by just emailing them when there heater died ...The preset heaters are no good IMO
especially if you have a tank 10g or bigger as it does not have any way to account for tank volume.
There is a page in the directions above that will tell you how much an Aqueon pro can raise the tank temp above an average room temp of 68-72 by size of tank and heater.
It is the most useful way to know what size heater you really need.
I by all my heaters on line usually from amazon or Ebay for well under 1/2 the price stores sell them for .