Keeping my goldfish tank safe?

Question
Hi there, I inherited a 20 gallon tank from a friend with two goldfish (1 ryukin, 1 comet), and then I upgraded recently to a 37 gallon eclipse tank system. However, when I upgraded I was encouraged by the pet store to add 2 more fish (also ryukins), and now after doing research I'm finding that it's probably crowded, at least for the long haul. The comet is at least 2", and the ryukins are probably 2", 1.5" and 1"? The older ryukin and comet seemed to grow quite rapidly in their first tank :)

Anyway, I did just drop a fair amount of $$ on this setup, and I like all the fish and am not keen on taking one back, so my questions are:

1-How long is it reasonable to expect that this tank will hold out on these fishies before I need to upgrade again or move one of the fish out?

2-How often and at what percentage should I be changing out the water for this many goldfish in this size tank?

I've only had them set up for about a week, but when I put it together I moved over more than half of the existing tank water as well as most of the gravel and an aerator (is that what it's called?). The Ph and Ammonia levels seem to be fine, ammonia=0 and Ph is between 6.6 and 7.0. Temp is slightly cooler than room temp (I haven't hooked up the heater yet). The fish are usually fed sinker pellets because I had a problem with the original ryukin getting swimmer's belly.

Thanks for your help!

Answer
Hi Rachel,
Well its great you upgraded and your goldfish will do fine in your 37 gallon for quite a good while. Their growth rate depends upon their level of care,feeding and genetics too.

As far as water change schedule. This depends on your nitrate levels. Testing your nitrate level is important and should be done before each water change to make sure your current routine is sufficient. You should aim to try to keep your nitrates below 20. It can be difficult to do that with fish that produce a high bioload like goldies and it may mean that you have to change it two or three times a week to keep up. Long term exposure to high nitrates can really do damage on fish and can even stunt their growth.

Try to vary their diet with some frozen foods and vegetables to help the one ryukins swim bladder tendency from happening too often.

Best of luck and I hope this helps!
Karen~