Goldfish suddenly all swimming at top of acquarium

Question
QUESTION: I have five fantail goldfish who were all happily utilising the whole of my large aquarium and for two days now are all swimming at the top.  I fed them tonight and they don't seem interested in the food.

The only change I made was to change the filter motor as the other was playing up.  The water is very clear and I have tested it and its perfect.

Could I have overfed them and maybe they are constipated?

Also how many goldfish should I put in my tank which is 1.5 metres wide, about .5 metres high and .5 metres deep.

Thank you so much for your help
Kind regards
Mattie

ANSWER: I calculate the tank to be about 99 US gallons or 82 UK gallons.  That's a good size tank.  In order to best answer your question, I have more questions.

How long have you had the tank setup?
When did you last add a fish?
What kind of filtration and aeration do you have?
What is the tank temperature?
Are there any other plants or animals in there?
Have you tested for pH, hardness, alkalinity, ammonia, nitrite, nitrate, and oxygen?  If so, what were the results?  If not, I suggest testing for those to rule out water quality and oxygen problems.
What are you feeding them?
Do they have any other symptoms?
How much water do you change and how often?
What do you add to the water?  I suggest dechlorinator and a tablespoon (14 cubic cm) of aquarium salt per 5 gallons.

Fantails may hang, especially at the top for a number of reasons - temperature, illness, digestive problems (gas for example, not really constipation but it's possible), poor water quality, low oxygen, and more.

If the problem is digestive, then feeding boiled, peeled peas may help.

Good luck!

---------- FOLLOW-UP ----------

QUESTION: Dear Robyn
Thanks so much for your prompt and comprehensive reply.  To answer your questions:  Tank has been set up for 3 months.  All five fish are my initial fish as I haven't had an opportunity to buy more fish.  I have  a Juwel tank which came with its own internal filter and heater.  Temp is 22 degrees C. Aeration - this I don't have.  I did have plants but they were not surviving well and took them out.  Did water tests for all listed except oxygen and everything was perfect.  How do I check for oxygen.  This morning I decided to do a 20% water change even though I had done it a few days earlier (I change 20% weekly).  The fish suddenly started utilising the whole aquarium.  However tonight three are again at the surface and two swimming around.  I now suspect it may be an oxygen problem.

THe fish seem healthy.  I give them a mix of pellets, flakes, peas, zucchini and corn.  

I add water conditioner to the water:  stress zyme and a bacterial solution.  Haven't used aquarium salt yet.

How many fantails should I house in this size aquarium.

Thanks again for your help.  You have been excellent
Kind regards
Mattie

Answer
I'm not familiar with teh "Juwel tank."  I just looked on-line.  The tanks are in the UK where you must be.  They seem to have a sump-type system.  The filtration should be ok.

I suggest absolutely adding a nice air stone attached to air line tubing to an air pump to aerate the water more.

Here's an oxygen test kit sold in the US.  I don't know if you can find it in the UK.
http://www.drsfostersmith.com/product/prod_display.cfm?pcatid=4366&Ntt=oxygen%20

If the fish are more active after the water change, the water may have been low in oxygen or stale (high organic levels which you didn't test for).  I suggest putting fresh activated carbon in the filter; it removes excess organics if they were an issue.

It does sound like oxygen may be the problem since the tank's not overcrowded (resulting in poor water quality).  

Adding aquarium salt at a tablespoon per 5 gallons will really help with a lot of things.

Ideally, you would not keep more than 10 goldfish in your tank.  I wouldn't add more fish until your current fish have been acting well for a while, and the water tests fine (as far as the cycling goes, no ammonia or nitrite).

Enjoy your fantails!  I have three of them in my 65 gallon tank.