Dying swordtails - lying on ornaments & bottom of tank

[fishfriendly]

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.

[Adrienne]

Have you added any fish lately?

[Jennifer]

Gill problems usually fall into the following categories: chemical toxins, parasites (worms, flukes, protozoa), bacteria or fungi.

You say that you see the gills are red, are the fish breathing very rapidly compared to non affected individuals? Are the fish rubbing up on anything? Did you have a close look at the gills of the dead ones? If so, what did they look like? Any white spots, white threads or swellings?

Sometimes it is easiest to treat for some of the more common causes of respiratory distress. Gill flukes can be eliminated by treating with praziquantel. Columnaris bacteria can be controlled by treating with an antibiotic prescribed by a vet.

[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 --