You wrote "I cleaned out their fish tank (30 gal) after not having to in a year".
When you say you cleaned out the tank what was done?
Did you change some of the water, all of the water?
Also, how many fish are in the tank?
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Thank you for the information.
When you changed all of the water you effectively made the tank a new tank and as a result caused the tank to start the cycling process.
The fish's lethargy and bottom dwelling is being caused by the presence of ammonia in the water. Ammonia is harmful and lethal to fish as is a normal occurance in a newly set-up tank or a tank that has undergone a large or complete water change such as your tank. Ammonia is caused by fish waste and decaying matter and in an established tank will be broken down very quickly but in a new tank it won't.
Listed below are 3 links that will explain cycling, I suggest you thoroughly read them and follow my recommendations below.
In order to get through this most difficult period in the life of a new aquarium and save the fish certain steps should be taken to minimize the impact. They are:
1) Drastically cut down on the amount of food being fed as the excess fish waste produced by overfeeding is fuel for ammonia and nitrite growth. Feed the fish a pinch of food once every other day.
2) During the cycling process do not change or clean the filter nor the filter media as this is where the beneficial bacteria will grow.
3) Do not add any fish to the tank during the cycling process, even if one dies. The only time you would add a fish is if all the current inhabitants die.
4) I know it's a temptation but it would be best to not administer any products that eliminate ammonia and nitrites as these products are only a temporary fix and will delay the cycling process.
5) Test the water every 3-4 days and when either ammonia and/or nitrites begin to rise immediately do a 20% partial water change. Water samples can be brought to your local pet store for testing or you can test on your own using a standard test kit(s).
6) Add some aquarium salt to the water at a ratio of 1 tablespoon per every 5 gallons of water. When doing a partial water change add only the corresponding amount of salt to the water being added. The aquarium salt will help fthe fish cope with the nitrites.
7) Keep the water temperature in a range of 62f-72f degrees as this is the preferred range for goldfish. Also , water holds more oxygen at lower temperatures.
8) (You didn's speak of ph but just to make sure)--Do not attempt to adjust the ph at this time as changing the ph will further complicate matters.
Best wishes and please let me know if you have any questions and please do try and read the links listed below.
http://www.thetropicaltank.co.uk/cycling.htm
http://www.bestfish.com/breakin.html
http://www.aquahobby.com/articles/e_syndrome.php
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