They will definitely outgrow the 55 in short order. I would move them to the 75 regardless and in a couple of years (depending how much you feed) odds are you won't be happy with them in a 75 tank either. I have found once the haps get beyond 7in they really look cramped in a 4ft tank to me.tmoore said:I currently have 6 Placidochromis Phenochilus in a 55 gallon tank. I was wondering what else could I house with them? If need be I could move them to a 75 gallon if thats what I need to do to be able to add to them. Bring on the suggestions.