Rust color on sides of tank

Question
Hi Jaymie-

I have had 3 goldfish in a 10 gallon tank for about 2 years.

I get this brown colored coating on the sides of the tanks about a month or so after the water change, it starts out as a few small spots then within a week it is all over the sides in splotches. I can easily wipe it off but within 2 weeks it is back. The water is still clear it is just the stuff on the sides. I used to do a 1/2 water change when I first got the tank and the brown color would appear with in a few weeks so I do a full water change and the brown color doesn't show up quite as quick. I use a water treatment at the water change and whenever I add more water.

Any idea what this brown stuff is and how to prevent it?

Thanks,
Jim

Answer
Jim,
Your tank is grossly over stocked. Here is why I say this. The absolute smallest tank that one goldfish can be housed in is 20 gallons, with 10 gallons for each additional fish. To house the three you have, in the absolute smallest tank is 40 gallons. The filter you should have in that 40 gallon tank should be for a larger tank. The goldfish does not have a stomach and everything that it eats comes right back out as ammonia. If not taken care of it becomes nitrites, which is very deadly. My suggestion is to try to get them the largest possible tank you can. Now for your question. You are seeing aglae on the side of the tank. This can happen if you have the tank sitting by a window, or if you have their light on too long during the day. You can curb this problem by shortening the amount of time the light is on, or by moving them away from the window. I see, however, another problem. You should not be doing 100% water changes unless you do not have a filter. If you don't have one, you should get one. If you are using a filter, everytime you change out all the water, you are setting the filter up to be cycled. This is something you only want to do once because it take 6 to 8 weeks to do. Once the filter is cycled, it holds beneficial bacteria that the fish need to stay healthy and the filter needs to keep the fish healthy.