The plecos haven't caused a problem with the goldfish as of yet and everyone tells me not to match goldfish with anything besides goldfish but this tank has been up and running for about 6 months now and this is my first major problem with it. I was going to change it up to a canister filter as soon as I can find a good deal on one but I have to experience with canister filters nor this sponge filter thing. I've always had the h.o.b. filters.
Goldfish
Buy Prime water conditioner if you don't already own some. Buy a master Test Kit. Test your water before and after the water changes.
Do a large water change, 50-75%
Make sure the new water is same temperature AND put Conditioner in the new water before adding to tank so you don't forget. Prime is 2 drops per gallon and you can add extra to get rid of Ammonia without any harm.
Keep changing water till Ammonia is a safe level.
5 Goldfish need a Minimum 60g-100g to stay clean and thrive. If they are Common or Comets then they need a Pond. They need 2-3x extra filtration. A 60g should have a filter rated for an 100-150g for example.
Goldfish get BIG, they poop a LOT and they can live a LONG TIME with good care and proper sized environment.
Fancies can live 15+years and get over 6" body length without tail
Comets can live over 25 years and get 10-12" or larger.
The record for age is 43 years
The record for length is 18.9"
6 months is nothing in the life of a Goldfish
Sadly I predict you will have more problems unless you open your mind and are willing to keep learning.
Plecos get huge and poop huge. Common Plecos can reach 24" and easily over a foot. I know of many Goldfish injured or killed by Plecos.
Keeping Goldfish warm makes their metabolism speed up, shorter life, poop even more. Less quality of life. IME and a vast majority of long term experienced successful aquarists.
It's your tank, but I hope you chose what is best for all your fish long term.