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.