fishfriendly

i havent added any new fish lately -- i only ever take fish out as they get bigger --- Fish do appear to be breathing more rapidly than unaffected fish, affected fish also have clamped fins, and have to swim quite hard to stay almost stationery. I havent studied a dead ones gills closely but i will study the next victim with a magnifying glass. the fish struggling dont apear to rub themselves on anything they just start lying down, even on top of my filter pump. I also put in an airstone incase they needed more oxygen but that made no difference. Tank has inbuilt filtration system in hood of tank. I did think my large main breeding swordtail had quite long mucousy poo yet this fish still seems very happy. When we did the large water change there was a bit of black hairy algae stuff on some plants and a few bits on glass of tank which we removed and i took the few plastic plants and 2 real plants out to get rid of it incase that was a contributing factor. We have 4 other tanks all running great its just this one that is playing up --

Dying swordtails -- hi i have a 200 litre tank which i use to breed swordtails and bristlenoses in. this tank has been set up for a year and none of the swords i have bred have ever died in it before. 4 days ago i notice a swordtail lying on an ornament (on its stomach) it was finding it hard work to stay in one place. next day it was dead and i noticed a few more lying on bottom of tank and on ornaments -- they died. this is still continueing. the strange thing is it is my swords that are about 2cm in size that are dying so far -- i have newer ones in there only 1cm and 3 full size ones that i breed with and they are at present okay. the bristlenoses also appear fine. I gravel vac every 8-9 days (usually 20%) but i have done a 40% water cahnge to see if it would help -- but it didnt. I have tested ammonia, nitrites and nitrates and they are nil - as low as you can go on my test kit (as normal) and ph is the same at 6.6. so what is killing my fish -- i can see some more getting red around the gills and suspect they may be next in line to go. Could it be parasites, worms ???? i wondered about bacteria but it seems odd that the bristlenoses are okay, although there are only 2 adults and about 10 1.5cm babies. Tank definitely isnt overcrowded -- infact it looks too empty. Where do i start to try to fix it - there are no noticeable signs on the outside of swords to spot a problem, they look the same as normal but some have red gills. The only disease i have ever delt with is white spot and that was months back - these fish werent around then but at least i knew how to treat it. Any suggestions !!!!! its now monday and now even my main breeding bristlenose is sick - still no outward signs ie worms spots etc so i figure i might as well try treating for something before i have no fish left - thought i would start treating for bacterial problems so what do i use -- any suggestions, do i try that maracyn & maracyn 2 stuff ??? note - no new fish have been added to tank - this has just happened for no obvious reason that i can see.