Harlequin With White Lumpy Fin?

Just over a week ago we got three new harlequin rasboros to replace our kamikaze ones that seemed to enjoy leaping out of the tiny holes in the lid.  Made our harlequin count back up to five.

A couple of days ago I noticed that the smallest of the new ones had a white and lumpy 'side fin, (I'm sorry, I'm sure there's a proper name for it).  I don't recall it having that when we bought it, but I may just have not noticed.

It doesn't seem to bother it.  It's swimming around happily, it's eating OK, and as far as I can tell is otherwise healthy.  Although it doesn't seem to move that fin normally.  I can't see anything similar on any of the other fish (neons, corys and two snails).

I've tried to take photos, but it's really hard to take photos of fish that are zipping around all the time isn't it!  So it's really blurred, but I think you can see the bright white area I'm worried about.

Any ideas or advice?

It is quite hard to tell from the photos, so some questions.

Does the lump look like it is inside the fin or on the surface?
Is it an amorphous blob or does it have sharply defined edges?
Does it look as though there is something inside the lump?

It looks like it starts on the body, behind the fin, and extends to the fin itself.
I'd probably go with an amorphous blob, but it's difficult to tell, because it's so active.
I don't think there's anything inside it.

Also, it doesn't appear to have gotten any bigger since I first noticed it (which I guess is a good thing).

I'm sorry about the photos!  I tried for a good twenty minutes, but I'd just fed them, so they were a bit giddy.  I'll get the proper posh camera out, and try tonight when they've calmed down a bit.

I'd be interested to hear that Sue even if Rachel's fish doesn't have a fungal infection

Right.  I spoke too soon.  One of the others has it a little bit on both side fins.  He looks less happy about it, and is constantly flicking those fins, as though it's uncomfortable and he's trying to shake whatever it is off.  His looks a bit different though.  Still white, but this time it looks as though it starts on the very tip of each fin.

So.  I've now done a water test, and now I'm panicking about that as well!

pH 6.0 - it's never been this low!  Usually around 7.6 ish!  We have very soft water around here, and I've posted before about how surprised I was about the high pH in the tank, but now it's crashed!!

Ammonia - 0-0.25. It's been stable at this level for almost a year according to my little book
Nitrite - 0. Again, stable at this for nearly a year.
Nitrate - difficult to tell because the colours are so close, but 40-80. Not usually this high.  Usually about 20

All I can think that I've done, is get a new load of filter media, but I added it to the stuff that's there, I didn't remove any.  We used to have a shell, and now we have a plant - would that make a difference?  And we have less algae than we used to.  I can't think of anything else that's changed.

I'm going to go and look at some photos of fish fungus to see if I think it looks like the culprit.

Bad news.

The one I noticed yesterday that had white bits on the end of both PECTORAL (thanks Steve!) fins, has died.
He did seem to be a bit distressed yesterday, but was still swimming OK, and eating fine.  But today I've come home from work, and sadly he's died.

Everyone else seems OK, including the little one with the white lump.  So I'm hoping it's a one off.  I'm a bit sad because he's the first of my fish that have died from non suicidal causes (I've had three rasboros leap out of the tank, but I think all gaps are now sealed so they can't do that any more).

I'm going to do a small water change after tea, and every day this week, and hope that whatever the water is doing, it can get back to normal.

Sorry to hear that.

Sometimes fish are sick when we buy them. They are very good at hiding the fact they are sick as sick fish get picked off by predators, and some shoaling fish even kill sick members of the shoal to avoid attracting predators. And with fish farms being big business, they don't have the time - or incentive - to remove sick fish before shipping them out.

When you go to buy more, study the fish thoroughly before buying.