My fish has not been eating.

RoseMadder

We have this fancy fish that we have loved and cared for, for 5 to 6 years. We love him very much and would hate it if he died but I think it is about to. I think this is to late but I am still giving it a last shot.
Two days ago my father cleaned the fish tank we usually take all the water out and put new one in clean the grovel and little houses. We have done this for the past 7ish 8ish years.
Anyways my dad cleaned it and put the fishes back in we have about 7 Goldfish including our big fancy fishes that is about 9 inches long and also 3 other tropical fish. I don't know the name but the are skinny and have black lines they are grayish in color well coming into the water the three died. We thought the water was to cold but it wasn't. Then we started noticing that the biggest Goldfish started to act funny. like if it was going to die. It didn't BUT it was been suffering for the two days.
These are the symptoms:
He has been in the bottom of the tank
Hasnt gottn a bite to eat
The bottom of his belly is pinkish red
Is VERY bloated
And is gasping for air
It is the saddest thing I've seen, It makes us all very sad and uncomfortable since we cant find what to do. We tried giving him peas but he would eat them, I had read they are good for constipation (which I am positive he has). He wont eat the peas. and at the moment he is laying on his side so he is on his last leg. I dont know what to do and I think he will wake up dead but if he doesnt I would really love it to find some answer tomorrow morning written here for me to try and apply. Maybe some way for me to force him to eat? Because he absolutly will not eat.
Please help, THANK YOU

Our tank is 20 gallon, We dont use test kits sorry!, the water we use is tap water and we put the aquasafe water conditionerwith bioextract., aquarium has been set up for about 7ish years.,We have about we had 9 fishes minus the three that died. The biggest is nine in. and the rest about 3, We have had them about 5ish years, the younger ones about two to one. None have been put into quarantine, the water is room temp., There are no plants in the tank, We have a power filter, We also use the oxygen bubbler, It receives plenty of sunlight, We changed the water two days ago and it was all, I know you arent supposed to....I knew this till now but we've always done it this way and none have died this way, We change the water everytime it gets pretty dirty almost the month, We clean all the gravel from the bottom, We feed the normal flakes once in the morning and once in the evening, there have been no unusual sings only the ones described,

kitten_penang

maybe the chlorine levels that day were very high.so sorry to hear you fishes died.maybe you need to add and extra aeration pump to get the oxygen levels up and add a little aquarium salt to help with the stress and breathing.they turn red when they are in chemical shock.try adding a little boiled (room temp) or RO water to dilute what ever chemical thats in high levels in you tank. hope it helps.

Bacchus

the tank is waaay overstocked...Goldfish are messy fish and produce a lot of ammonia. you may want to do a large water change, keep a small sample, and then take the sample to the fish store and have them test it for you. i would take back all the others and think about getting a slightly larger tank, if the fish didnt die already