When a fountain pump goes bad, it is most likely the impeller is stuck or broken.
Most fountain pumps nowadays use magnetic drive systems. Inside the outer casing of the pump is an impeller with a magnet next to the impeller's fins. Inside the inner casing is another magnet. The magnetic force from the motor’s magnet will cause the impeller magnet to rotate the impeller.
Sometimes a fin or fins break off, but more often the cavity in which the impeller fits gets too much crud or slime in it and the impeller gets stuck or has a difficult time starting to rotate. The magnets can also get stuck in shipping. To find out if you have an impeller problem, you will need to open the outer casing of your pump.
Most small fountain pumps have a front plate that snaps off. For larger fountain pumps you will need to remove screws holding the outer case in place.
We are including links to videos here on how to inspect all Fountain Tech impellers.
How to clean or inspect a small pump impellers
How to inspect a Fountain Tech 1900, 2500, 3500, and 6000 impellers
How to Inspect all All Fountain Tech impellers