11-01-2010, 01:58 AM
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Four mollies in a 75 gallon tank would prolly not create much in the way of stressful or dangerous levels of ammonia or nitrites and the borrowed bacteria from your thirty gallon tank is helping also.
So long as you perform weekly water changes and do not add more than two or three fish every ten days, I would feel comfortable were it me.
Would NOT add any more than two or three fish every ten days to allow bacteria to catch up with the increased Bio- load on the filter and bacteria contained there for at the moment,,, the bacteria has developed for the waste created by four mollies and the food being offered. To add TOO many fish at once,or too large of fish at once ,will most assuredly result in increase in ammonia and nitrites that may rise to dangerous levels in a matter of a few hours.
Most of the benefits you have received thus far, are due to the borrowed filter from the 30 gallon assuming it was running at the time you removed it and placed it on the 75 gallon. There is very little bacteria in the used water from a cycled tank for the bacteria ,or bio-film as it is sometimes called ,,adheres to solid objects such as decorations,wood,plants, glass,and gravel or substrate.
Would add the fish as described were it me, and feed the fishes sparingly perhaps once a day.