What exact size is your 'large' tank. How large is each fish. Check water ph, nitrate, nitrite, ammonia levels?

Because if not playing??? one sounds like he might be weak and others will pursue a sick fish. But not sure, more information is needed. Goldfish need about 10 gallons per fish (this of course depends on size of fish, more the better) or more when small, the more air surface the better. Are they goldfish or koi which grow very large. They need super filtration. So check back with more information, ok.

PS. Golfish really like to 'mouth' gravel, ie pick food out from it, not to big nor too small. And especially they need and crave live plant matter. So if no real plants for them to nibble on, feed them green blanched (cooled) vegetables sans skins (peas for instance, zuchinni so on)

Best Sherry

Equipment & Maintenance:

To keep a goldfish healthy, remember to provide them with plenty of clean, cool water.

Your goldifsh's temperature should never rise above 73° Fahrenheit, and ideally should remain between 65° and 68,° so you will need a thermometer. Keeping a goldfish above 72° for long periods of time will result in Oxygen deprivation, which can cause nerve damage, heart damage, and can seriously hamper the immune system, making them more susceptible to many diseases, from easily treatable Ick to nearly untreatable swim bladder infections.

Keep your goldfish in a tank of at least 10 gallons, and make sure that a minimum of 8 gallons of your tank space is dedicated to each goldfish (a 10 gallon tank is really only suitable for one goldfish). This will provide sufficient space for your goldfish, so he can remain healthy, and this will allow the tank to help you keep healthy fish.

Perform your weekly 10-15% water changes, so that your goldfish always has clean water and the waste that is not processed by the filter is removed. Your goldfish will need a filter to keep the water from becoming toxic, and you will need to keep the filter in good condition.

When selecting a filter, if you have fancy goldfish, make sure that the filter will not produce excessive current in your tank, and this will make it difficult for the fancy goldfish to swim.