sounds to me that your tank may be like a time bomb waiting to explode ... mushies are very hardy, relying on trace lements and lighting. while you see them opening up big now and spreading, can be very fast that you see them shrink and melt. have you checked your parameters already ? more so, you are adding 'tap' water, you will do better with distilled at least. peace already posted the link on tank cycling, so it's best to read through. also, first thing's first, check your params. just my 2 cents. cheers !