Real Newbie Question: What's that Urine Smell

Here's the set up:

I have 5 goldfish, (two are Black Moors,) in a new 5gal tank.

The tank, (from Petco,) is a Aquatic Gardens Five Gallon Mini-Aquarium. I set up last weekend: rinsed everything, filled it, treated the water, transferred the fish from a temporary 2.5gal tank when water temps matched.

Tank has no heater, but the 15watt bulb puts out a lot (too much, IMO) heat. The hood is fully enclosed with thin ventilation slits. There's also a removable clear, flat plastic cover that lies on top of the tank -- under the hood -- with a slot to feed the fish.

Here's the situation:

When I entered the room the tank is in yesterday, there was a very strong urine smell. At first I thought the can't left a gift, but when I lifted the hood on the tank, it was obvious that was the source of the smell.

Can someone advise me on the source of the "urine" (ammonia?) and how to handle it.

Thanks!

RockWalk

Not to argue capicity of fish-to-tank, but to clarify:

The 3 "standard" goldfish are 1" long; the 2 black bubble-eyes are 1-1/2". The bubble eyes lived in a temporary 2.5gal tank for 3 weeks without a problem.

We have another 7gal tank with eight smaller fish (differing varieties,) all 1"/less in size. This tank has been going beautifully for over a year.

We're falling the same program with the new tank.

Any suggestions? Is keeping the 5gal tank a viable option? If so, how can I adjust the environment to improve the conditions?

Thanks again!