What do I do about this growth in my tank?

I seem to be having the same problem as you, I'm not sure what it is... but I can tell you this that goldfish should NEVER be kept in that small of a tank... they need atleast a 30+ gallon aqaurium.... and your 5gallon is WAY OVER STOCKED!...

I'm starting to do some research on the brown spots... when I find out about them, I'll let you know...

Alright, here's the answer to our problem.. hehe...

BROWN ALGAE
Other Names: Gravel algae, Silica algae

Color: Brown

Appearance: Begins as brown patches on the gravel and/or glass, then rapidly coats most surfaces of the aquarium with a thin, dark brown coating that is easily removed. Unlike blue-green/slime algae, it does not come off in large slimy sheets.

Cause:
Excess silicates & nitrates

Inadequate light

Low oxygen levels

Brown algae is a common occurrence in a newly set up aquarium. It is generally caused by too little light, an excess of silicates, an abundance of nutrients, and too little oxygen. Silicates can build up through tap water that is high in silicic acid, and silicates that leech from some types of substrates.

Cure:
Wipe off surfaces & vaccine gravel well

Use silicate adsorbing resin in the filter

Increase the lighting

Stock a plecostomus or several otocinclus

This type of algae does not adhere strongly to the tank surfaces, and is easily wiped away. Vacuuming the gravel with a siphon will quickly remove coatings from the substrate. Increasing the lighting will inhibit regrowth of brown algae. As a new tank matures brown algae is often eliminated naturally by plants and green algae competing for nutrients.

Some suckermouth catfish will readily eat brown algae, most notably plecostomus and otocinclus. If the problem is due to high silicates in the water, and the brown algae persists, a special silicate absorbing resin can be used in the filter.

Prevention:
Use of RO water

Regular water changes

Regular aquarium cleaning

Good lighting

As with any algae, keeping the tank clean and performing regular water changes is one of the best preventative measures. Unfortunately it is still possible to get algae in spite of regular maintenance, especially in a newly established aquarium. Prompt attention to sudden algae growth will prevent more serious problems.
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I got that information from this site: http://freshaquarium.about.com/cs/maintenance1/p/algaebrown.htm

I think you really should bring back some of your fish, you can't keep coldwater and tropicals together in the same tank... but also maybe you should think about upgrading to a bigger tank...

The brown spots are a normal stage in cycling a tank. As a cycle settles in the brown will disappear....However... It can become a nuisance in an overcrowded tank...and your 5 gallon I would say is definitely a tad over stocked...

You might want to consider a larger tank for all those fish....maybe a 55 might be a good start.

Yes...goldfish like a 20 or 30 gallon tank to themselves, and a single angel should have at least 10 to himself.

Also, either your goldfish is sweating his scales off or the angel is shivering all day long. The temperature preferences of these two are completely opposite.

Tank advice asked, tank advice given...same thing many others here will tell you.

Good luck with the mixed tank.

A 'common' goldfish can grow to over a foot in a good environment and live more than 15 years. Those little ten cent babies you bought as a kid and kept in a bowl lived only a fraction of their possible lifespan, dying before reaching maturity. Goldfish are better in coldwater ponds or very large tanks 55g or more. Even a 20g could probably house one blackmoor for maybe a year before needing double that. People do it in smaller tanks, but its all a matter of being ethical and responsible and knowing what conditions your fish come from in nature and providing as close to as possible the best conditions so they thrive, and not just 'survive'. This is the interesting part of fishkeeping. Have fun learning its a gr8 hobby.

I used to have a common goldfish who was longer than a 5 gallon tank, so I definitely second that they can't be kept in them!

The fancy goldfish don't get as long, but they get much rounder and still need the room.

The tiny goldfish that are sold in stores are basically "babies" - after 10 years, they definitely need a bigger tank.

Also, angelfish get quite "tall" and really need tanks that are 18+ inches high.

As for the painted glassfish, since the color is achieved by dyeing, the pigment doesn't last forever and often harms the fish, which might account for them changing color.

Generally, think of fish salesmen the same way you'd think of a used car salesman - they'll say what you want to hear to get you to buy their product ;-)