Can tank get too small as fish grow?

Question
I had four goldfish; three that were about 7" mouth to tip of tail,one about 5 inches and also a small 3-4" algae eater.  One of my goldfish just died for no apparent reason.  He looked fine in the morning then around mid-afternoon he was belly up but breathing and died several hours later.  I did a 20% water exchange and filter change the day before.  The only thing i noticed was that my filter dial was on "slow" (for feeding my fish brine shrimp) I normally turn it back on "normal" and I had forgotten so it was like that for about 20 hours.  I immediately turned it back to normal.  I also noticed that since I've been feeding these guys the frozen brine shrimp (which they love) they don't seem to go for the flakes and pellets as much now.  

Would that be the reason or do fish die off as they grow if the aquarium stays the same size (I have a 29 gallon low tank)  

Answer
Hi Amy;

They are pretty big for a 29. The fish that died was probably just the weaker of the group and died from oxygen deprivation. With the filter turned down it was less than they all need to survive. Or, he reacted with stress to the water change. Sometimes we never know why a fish died. They can have heart attacks and strokes and things like people do. As long as the other fish look okay, I wouldn't worry. Just don't add more fish. It's already overcrowded.

They would all do much better in a 50 or 55. For now, change 25% of the water weekly while vacuuming the gravel and feed sparingly. Fish really like the shrimp and yummy foods like that. They turn their noses up to less appetizing foods when the shrimp is available. It is normal. Just don't give in and feed only the shrimp. Keep making them eat the balanced diet of the flakes and pellets. Maybe feed shrimp 2 or 3 times a week as a treat. Goldfish should also have cooked green peas, chopped spinach, slightly cooked and shredded carrots and romaine lettuce 3 times a week. Don't overdo it, but give them tiny bits until you can find out what they like. I usually feed them along with flakes and pellets. These foods keep their digestive systems "going smoothly", if you know what I mean. ;-)

Followups welcome

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Chris Robbins

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