why does my fish tank filter keeps overflowing?

The media is likely clogged and the filter is not strong enough to push water though it. Goldfish need very powerful filters to handle all the waste they produce. If its not strong enough the water will not be able to push the water though the media and it will either overflow or run back into the tank near the intake.

Clean the media out (in tank water) to fix it but you'll likely need to buy a stronger filter as the goldfish will grow and will get messier.

Those filters are worthless! One came with a 20 gallon and I threw it out! It's the filter media that causes it. Make sure it sits level. AquaClear are the best. You can even put your own media in unlike Marineland.

With goldfish you should have twice the filtering also. I would recommend an AquaClear 50. That's 200GPH or even a 70 which is 300GPH.

the pad is restricting the flow, that's all. there's no problem with some water bypassing the filter pad. don't overreact when things like this happen.

you probably overfeed and food clogs the pad within a day. feed more sensibly and it won't happen as fast.

interior the cartridge there would desire to be algae/plant matter blockage. actually attempt the two changing that or a deep sparkling. alternative is right. I replace my filter out a minimum of as quickly as each and every 2 months. The cartridge, the bio-wheels are good for a protracted time (in case you have that variety).