Hi I have a large tank with 20 fish...2 being large oscars and the rest goldfish, shubunkins, koi's etc, they have all lived together for over a year, since the Oscars were very small, 2 months ago, 1 of the larger goldfish looked like she had bruising on her under belly, which then went away... but 2 weeks ago, started to return, this week it's much worse and very much larger, she also has some on the top of her head and some that actually look like wounds ? all the other fish are fine , and I have been watching them closely to make sure the Oscars were not getting heavy ...she stays alone , and is slow, sometimes eats? any suggestions guys

Hi Seawitch
   I know that the fish should be parted, and if I thought there was room for concern, I would have, as I have another tank too, this one is 6'x2x2 and holds around 120 gallons, so it isn't too small, the fish all are good together, as I said the Oscars came when they were only an inch or so in size, the Koi, are around 6/7 " and were meant to go in the pond in late summer but as that didn't happen and our weather here in Ireland has been at it's coldest for decades, I felt it would be best to leave them in the house ..safe till late spring ? in fact all the fish bar the Oscars where meant to go in the new pond, and hopefully thats what will happen in a few months, they get 3 different types of food, koi pellets, Cichlid pellets, and goldfish flakes, they have all been together for over a year, and this is the first problem I have encountered, there is a heater so the Oscars are okay for temp, it doesn't seem to bother the coldwater fish that it's a little warmer, the pleco needs a little heat also..I have e mailed a pet store to take a look at the fish tomorrow,and run some water tests for me , if I find out what the problem is I will let you all know..TY

Oh Yes you asked re the filtration system, it was designed for marine fish, the under tank filter tank is approx 5 feet long and has 5 different compartments for the filter mediums ? I have always been told it is quite a good system