After Water Change Fish In Trouble Help!

hi, i am new to this forum this is also my first emergency, here are the specifics...

275 litre tank
external fluval 400 filter (2xammonia remover, 2xcarbon, 2xgravel hole media)
heavy planted
4 x firemouths
3 x angels
2 x neon dwarf gouramis
4 x julii corys
2 x algue eater (sucking loaches)
3 x clown loaches
1 x bristle nosed plec

i have had trouble with the flow of my filter for a while so took yesterday off work and upgraded my tubes from the external filter, i also cleaned the media with tank water and cleaned the tank and did a 25% water change, i added the correct amount of tap safe and i always do a 50% tap water 50% RO water, i tested the water this morning and it shows no sign of chlorine, the ph is as normal 7.5ph, the gh is as normal 10deg, the kh is at 6deg as normal and i have no ammonia or nitrite and 20-30 ppm of nitrate, i have searched everywhere on the net for help but its really weird behaviour my angles all have symptoms of inactive, trouble to swim, slight slimy poo, and almost look blind, the same with the fire mouths and the algue eaters but the corys and the neon dwarfs and bristle nose are all fine, my clowns have hidden today so far but that is normal, i have done another 25% water change with tap safe and half tap/half RO .... all i can possible think of is if by some strange way a for of metal has got into the tap water?? but i thought tap safe should prevent that?? the temps dont really change that much and stay at a constant 27c, i have had this tank and the same fish for 8 months now and i just dont know what to do except keep doing water changes???

any help advice questions would be really REALLY helpful, thanks

i just need to ad i do a 1/8 water change every other week and a weekly water check and nothing has been up with my parmeters, i did add 1 new ammonia remover and 1 new carbon when i did the media clean and i cleaned them first with tank water, none of the fish showed these symptoms before the yesterday i dont know why some of my fish are fine and others are just inactive and even sometimes on there side on the bottom, its quite destressing, it couldnt be the new pipes could it?? i mean they come ready to go dont they?? other than that i cant think of anything i have done differently and its such a rapid decline of the fish

thanks for your reply i am not sure if internal infection as the symptoms have happen within 24hrs but do you know of internal inf that has those symptoms? news from over the night i have lost 2 fire mouths and the other 2 dont look great but my angels are still hanging in there though there is no real change, the ones that also looked seriously like they wouldnt last the night the algue eaters look like they have completly recovered??? the julii look very active so do the gouramis and bristle nose, i still havent seen the clowns though i have carried out another 25% water change just this second and i am hoping that my angels and fire mouth pull through