Scissor Fin Rainbows Dying

When you say your water parameters are spot on, I assume you mean ammonia, nitrites and nitrates? What are your other parameters? Hardness, pH etc? The more information you can provide the easier it will be to help. Is your tank planted and what size is it? What's your tap water like and what is your water change regime?

Sorry Sue I meant the forktails and I think just found the cause 

I was at my sisters for 2 weeks and left my son looking after the tanks, he said he'd tested the water in the big tank and everything was ok BUT the idiot only tested for ammonia, he didnt perform a weekly water change and he didnt test for nitrates until I asked him what they were and by then the nitrates were 80+ so I think they've got sick because of that. They do need really pristine water