I have had the same school of harliquins for over five years. I keep nitrates at 10ppm and pH at 6.4. 110 gallon community with other peaceful schooling fish and a mated pair of angelfish. I have slowly lost one at a time starting about 3 or 4 months ago. I went from nine now down to five and one is swimming lopsided. Could this be old age? No other loses for a couple years now.