Popeyed Black Widow Tetra With Other Symptoms

One of my five black widow tetras is definitely sickening for something 

Yesterday, I noticed that this particular fish had poppy out eyes and one of its gills is reddish.  Looking at it today I think its body is a bit misshapen and its not a nice rounded shape like its brothers/sisters.  Also it appears to be opening and closing its mouth a lot, but it's not gulping air from the surface.  My water parameters are good - ammonia and nitrite are zero, nitrate was 10ish on Friday (I then did a 20 per cent WC), pH is 7.4 and KH is 2.8 (quite low I know, but pH seems to be holding its own, so haven't worried too much).  Since the tank cycled, I've been testing about every 3-4 days and the parameters have remained spot-on - even when more fish added.

Tank now has five black widow tetras and six glowlight tetras... oh, and four zebra snails (although these were only added yesterday).  None of the other fish seem affected.

At first I wasn't sure whether this BW was one of the first three (been in the tank about 10 weeks) or one of the two purchased about two weeks ago, but after giving it some thought, I'm almost sure that this fish is one of the original ones (so been through fish-in cycling, although amm and nitrite levels never got very high so hopefully not too stressful for the fish). 

Right from the start, this fish has behaved a bit "differently" from the others.  It tends to hang out in the top right hand bit of the tank and just stays motionless - so much so that during the first weeks of cycling, on several occasions I thought it was dead!  But it would then swim away....   As I'm typing this, it's motionless in it's usual corner.  The other four BW are swimming together happily.  It's definitely a different shape from the others, but I wasn't sure if this was just a difference in their sex.  Have tried to take photos but they're not very good and you can't really see the problems so haven't bothered posting them.

Do you think it's dying?  I've been trawling the internet and sometimes I think the symptoms of dropsy match it (it's body does look a bit "pine coney") but then with dropsy it says the gills are pale and the gills (certainly on one side) are pink/red.  I hate the thought of it being poorly, but I also dread the thought that it might give it's disease to the others.  I don't have a quarantine/hospital tank I could move it to and also no chance of getting a tank, filter, heater etc. before next weekend due to work commitments.

Sorry for the long essay but would really value your advice.  Do I just leave it and see what happens?  Do I get some kind of medication, but as I don't know what's wrong, which kind?  Should I euthanise it before it affects the whole tank - that sounds horribly callous to say that, but if I'm not able to isolate it from the others,  won't they all get it?  As I said, although I only noticed the poppy eyes over the last day or two, with hindsight I think this fish has been sickening for something for quite a while.

Hope someone can help....  Thanks!


Liz

Thanks for your reply Sue.  Unfortunately, today the fish seemed worse (certainly no better).  It had a long string of white poo attached to it and it looked very patchy, like lots of scales were missing.  It was also still breathing very rapidly.

After reading your advice and weighing up the pros and cons, I bought some clove oil and put the fish down 
Feel very guilty but as I think the fish had probably been sickening for several days before I noticed the poppy eyes, I was worried that by the time any medication was administered, it might have passed to the other fish.  Also it just didn't look at all happy.  I need to keep telling myself that!

I wish I knew what exactly the problem was - info on dropsy says it can be caused by stress and this particular fish always seemed a loner.  The other four BW would swim around together and more or less ignore the poorly one - apart from going in for a nip now and again.  Maybe it was a vicious circle - the poorly fish was ignored by the others which caused it be be stressed, it became ill, which then made the others be mean to it which stressed it out even more....   Anyway, it's too late now.  Hopefully none of the others will go the same way.  I did a 25% WC and so far so good.  I will be watching very carefully over the next few days.

Thanks again for your help.

Liz