I was fortunate enough to be given a tank about 4 months ago and I have or did have 4 goldfish now 3 as of last night plus 2 danios and 2 catfish which all live in a 136 litre tank which is fitted with undergravel filtration. I change 30% of the water once a week and the temp is around 17 degrees Celcius. I condition the water and my pH and nitrates/nitrates are adequate. AND All was well until about a week ago when one of my goldies houdini developed fin rot and then a fungal infection which i treated with MG and Methelyne blue, and now a salt treatement which has partially removed the fungal infection. But now all of my remaning golfish have stopped feeding (i fed them some peas yesterday) and 2 of them have moved to the bottom of the tank under one of my rocks and the other sits at the top of the water column. None of them have eaten anything today but teh danios and catfish continue to crusie around as if nothing is going wrong, Today I did a 50% water change today as the water was slightly cloudy and they look a bit happier can annyone suggest what might be wrong with my system or fish Any help would be greatly appreciated ) thanks Wilbur...

Hi Wilbur, and welcome to the board.

A few things spring to mind: firstly, your tank is somewhat overstocked; GF need at least 10 gallons per fish and your tank is approx. 35 gallons, with extra fish in there. As such, water quality might well be an issue here... can you test ammonia, nitrite, nitrate and pH and post the results?

Secondly, fish often don't react well to meds in the water - malachite green and methylene blue are actually quite strong and nasty substances in fact - so they may be reacting badly to these. If the water change you did perked them up a bit, this might well be the root cause of their lethargy. You could do another large change in a day or so, and add carbon to your filter in the meantime to help clear the water. Did you add extra aeration during the treatment, btw?

Thanks for your help emmahj my fish are starting to look a bit happier.
I have 3 airtones in the tank and i had them going at full speed, Ill get some N03 NO2, N and pH readings done tomorrow and post them for you. cheers wilbur

methyne blue will have SHATTERED your biological filter, watch out for ammonia and nitrite spikes.... also it will damage any plants u left in the tank when treating

its good for goldfish eggs though