sick pleco?

Question
i have a plecostomas that is about a year old, and 6 inches long. we have him in a 25 gallon tank with 1 fancy-tailed goldfish. they've been together now for the entire year, and don't seem to ever bother each other. the tank gets a new filter put in and washed completely every week.
the pleco has been perfectly fine up until the other day...
normally my pleco moves around a ton, whether it's day or night, and eats everything, and is always seen sucking on the glass.
the other night, he was laying on his side, and thought to be dead. when i took him out i noticed that he was still breathing, and put him in a 12 by 5 container with 3 inches of water under a lamp. i had the water checked at a local pet store, and they said it was fine. once in the containter, i took a closer look at him and i noticed that he doesn't suck on anything. not even his algae wafers or flakes. he only sits there. when you go to nudge him, he tries getting away, but flips upside down when he tries swimming. i have also noticed that his belly is starting to sink in a bit, which i assume was from him not eating anything. there's times where he flips on his back and stays there. when we try putting the broken up wafers in his mouth, he only spits them out. also, i have noticed that he had a white edging around the edging of him. the white part peeled off, and is now a bright pink color. his fins no longer stand up like they usually do too. his color is still them same. another thing i was able to notice was his tail fin is beginning to fray.
after checking him over i put him back in the original tank with the fish after cleaning the entire thing. it has been 3 days and there still has been no improvement in him.
we check on him daily, along with his food, and none of the food has been touched. we have been finding him in the corner, just laying there, for long periods of time, and occasionally on his back, seeming to be stuck, so we have to flip him back over.
when he does move, he only wiggles slightly.

Answer
Brittany,   This does not sound good. Your goldfish puts out much more wastes than a tropical fish. As he grows and they grow fast (your plecos) they make a mess themselves with strings of poop everywhere but, they are the only algae eaters that you can put with them The only problem is they grow out of the tank faster than your goldfish will and it will too. I don't know if he can be saved you can go and get a siphon and make sure you are cleaning them rocks well at least once a month. If you are not familiar it is a tube with a hose that you move up and down and create a suction into a bucket. Move it up and down in your stones to make sure your getting the solid wastes out. You filter and bacteria can only do so much and I would do a 60% water change every time. The thing with goldfish is you can't have enough filtration the bigger the better. Even though your water is ok.... at the bottom where your wastes are it will be stronger and burn your pleco. I would do a cleaning return him to the tank take your carbon out of your filter and treat with tetracycline for 7 days. I think it is one packet for ten gallons. Return your pleco to the tank and hope for the best. Also throw a couple shrimp pellets in now and then for protein for your pleco and you goldfish after your done medicating give him or her a frozen pea once a week. Run it under water give it a crack and let it eat....don't forget to remove the shell later. It is a natural help for constipation that the goldfish are so prone to. This is usually what leads to swimbladder disease or dropsy. It will also love if you get a veggie clip from the pet store (it is a clip with a suction cup) and give her a slice of orange once a week. Rind in meat out. If you look at her food it is enriched with vitamin C and this is common for captive goldies to have these deficiencies but this will remedy that naturally and only leave it in for about an hour. Get him back into a medicated clean tank quickly and he may just surprise you they are tough. If you have any other questions ask...Good Luck, Tina