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Twenty-gallon tank once housed 12 mollies, 5 serpae tetras over the past 4-6 years. The last inhabitant (except for two small spotted corys) died this morning, apparently of old age, as did all the others (no evidence of disease, deaths were spread out over long period of time). I'd like input regarding restocking the tank using the same water. I've read references to 'old tank syndrome' but rather unclear how it would affect new fish being added. (I do have an active 10-gal tank I could put the corys in if I need to change water, substrate, and recycle.) Thank you for any information.
 

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Welcome to the forum

Old tank syndrome is where the water chemistry of the tank changes over time to be different than the source water, through lack of water changes. This change occurs gradually over time, and the fish adjust to it. However, new fish are shocked by it and die.

It's best to keep your tanks water chemistry as close to the source water as possible, meaning performing water changes with some degree of regularity will prevent old tank syndrome.

There's no need to restart the cycle. Just change the water and add some fish.
 

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Do you mean change all the water? I do partial changes - approx. 1/5 to 1/4 - weekly, unless too 'busy" (lazy!), but never more than two weeks apart, and don't top off ttanks.

Reading your other posts - you are expert! - I'm thinking of restocking with all one speciies, like small tetras or danios. Would you recommend introducing all 15 or 20 into tank at the same time and from same source, or spreading it out a bit?

Thanks very much for your advice.
 

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If you did regular water changes then old tank syndrome shouldn't be a problem. It would still be a good idea to do a large water change before restocking. Periodically it would be a good idea, after you have fish too. Small water changes just don't clear out the water like the big ones do.

Ask me something about plants and you won't think I'm much of an expert :) I know what I know and I know what I don't know, and I don't often speak about things about which I do not know.

Because your tank is/was lightly stocked, I would only add a few fish at a time. You can add more fish at once to a heavier stocked tank, but adding too many fish to a lightly stocked tank will cause a mini cycle. Not that that's the end of the world or anything, but why not avoid it if you can?


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