55 gallon and twice a week water changes

Two maybe 3 tops for that size tank.

If you go barebottom, no gravel, and siphon the bottom clean each day, you might be able to add more fish, to 4 or 5 maybe, if you also have extra filtration.

If you are talking fancy goldfish that reach no more than 8 inches in size, 3 would probably be very safe in there without too much worry.

Not in my fish tanks. 30 gallons or more per goldfish, period.

I do whatever works. To elaborate, if you are tiptoed between keeping 2-4 fish in such an aquarium, use common sense and start with two fish. Does the tank look crowded, could you see any more fish in this aquarium when the fish are fully-grown? How about the nitrates, are they staying at a reasonable level with regular water changes? In other words, if you have to do water changes 2-3 times a week just to keep the nitrates below 30ppm, your tank is probably overstocked.

In essence, it is common sense that scientists/experts use to calculate how much of any animal can be kept in a certain space. That said, go with the smaller number of fish, and let them grow and prosper in your tank for a while… then if you feel that another fish could be added, you may do so at that time. I, however, would not put any more than 3 Goldfish in a 55-gallon tank.

DLeung10 said:

How many large fancy goldfish can be housed for life in a 55 gallon (4 footer) if I change 1/3 of the water TWICE a week ? No pH adjustments would be made.

Filtration would be a power filter (AC300 or Penguin 330) and a canister like a Rena Filstar XP2.

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I think you could have couple more goldfishs in a 55 gallons. I have 8 in a 46 gallons without problem. I only do water change maybe once a month.

Really? And how big are they. He wants to keep them for life, which can be 25 years and they can get to be 12-18 inches in length. 4 would really be pushing it for a fish that large. It is amazing how people never want to put a single oscar in anything less than a 55 gallon (75 is proper) and yet you can cram a hundred goldfish in much less even though they get to be a similar size. Obviously there is a aggression issue with an oscar but still.

TKOS said:

Really? And how big are they. He wants to keep them for life, which can be 25 years and they can get to be 12-18 inches in length. 4 would really be pushing it for a fish that large. It is amazing how people never want to put a single oscar in anything less than a 55 gallon (75 is proper) and yet you can cram a hundred goldfish in much less even though they get to be a similar size. Obviously there is a aggression issue with an oscar but still.

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Mine 8 are not that big yet. They are range from 4 to 7 inches now. I think 12-18" is bit high for a fancy goldfish. Do you have pictures to proof that it can grow to 18" and 25 years old in a tank.(in the wild maybe). The biggest I ever see is 10" inches long. You know, I had 4 ocars in a 46 gallons tank for 6 years, They have no problem.

Agreed, fancy goldfish do not get 18 inches long. They usually range between 6-10 inches. With this in mind, I still stand firm that more than 3-4 goldfish in a 55-gallon tank is really pushing the limits. These fish like to swim around, not having to bump into each other. They need pristine water conditions, or you'll notice discolorations on the fish, eye or gill flukes, and blood streaks on their tails. These are not beginner fish at all, which is why beginners should start with tropical fish first, in my opinion.

Are your 4 Oscars, which were in a 46-gallon for 6 years, still in that 46-gallon tank? And how large are the fish? Most would agree that to stick 4 Oscars in such a smallish sized tank for that many years would stunt the fish. Oscars, by the time they are 6 years old, should be around (or over) a foot long and quite hefty looking. Can you say the same for your Oscars? I'm just asking.

Thankfully, DLeung10 is looking for a permanent home for his fish… not a temporary situation with the idea of “upgrading soon”. I commend this person for that. I've said it before and will say it again, pets are not cheap, but they are our responsibility from the moment they were domesticated - we must take care of them, and must be able to do so from the beginning, or else one should wait until they can afford the extra 'family member'.