Bristlenose And Silver Dollars.

I have 3 Silver Dollars (1 is the size of a saucer & 2 about half the size)and 4 Bristlenose Catfish(all about 10-15cm long), (I also have other fish in the tank). We did a 3/4 water change because nitrate and ammonia levels were really high and have been for ages. A fight I am constantly losing. Anyway we also took our male and a female Bristlenoses out of the tank and left 2 females. A couple of hours later we heard a bit of commotion in the tank and when we checked to see what was going on, one of the smaller silver dollars was bleeding with a round hole in its side. The broken skin looked like one of the bristlenoses has sucked on it because it looked like it had been a huge lump that had exploded but none of my Silver Dollars had any lumps. I know they were all fine because I was doing a water change to try and treat one of the other silver dollars that had a sore on its side that was being affected by the yucky water and had made it turn white. So obviously I had checked all other fish to make sure none of the others were affected.

I have had the tank, setup and fish the same (apart from the water change) for about 4 months and never had any trouble before. Should I remove the Bristlenoses. I'm just curious if anybody else has had this...I don't want it happening again. I love my Silver Dollars....I also love my Bristlenoses. Do Bristlenoses suck on other fish???