Pygmy Cories - 2 With Missing Parts Of Tails, And 1 With A Spot & Bent Spine

NB. The title sounds more alarmist than the situation actually is.

Last week, I had the first tank fatality when a pygmy cory died after looking very weak and washed-out for almost a week. Since then, the tank has "livened up" (no grieving in this tank!) with the x-ray tetras' appetites back and both the x-ray tetras and pygmy cories very active.

I've now noticed that two of the remaining five cories have lost the bottom portion of their tail fins ie where the tail fin normally has two portions, the bottom one has vanished in each case.  This is a new occurrence, and I'm assuming it may be happening when they're pairing up/playing and chasing one another in groups of 2-3.  In both cases, it's "clean" ie there's no fin rot or anything else untoward.

I've also noticed that another cory, which had a white, slightly raised and almost pinhead-sized, dot on its caudal peduncle area (ie the portion just before the tail) last Fri but which disappeared the following day, has had this white dot reappear this morning. This particular cory has a slightly odd swimming action which I've noticed over the past month - at first, I'd put this down to overeating / swimbladder-like bloat from when the tetras' appetites were reduced (fantail goldfish had this in the past but which resolved itself after a laxative(pea)-induced 'clearout') but have recently noticed that there's a curvature along the stripe as though it has a bent spine - its front body is at a slightly different angle to its back body which tends to droop down.

I must emphasise that all pygmies are eating well and are very active - the two liveliest being Huge Cory who currently seems hyperactive and swimming constantly up and down the glass (which is one with a missing tail portion) and Bent Spine Cory (which is one with this intermittent white dot on its caudal peduncle), and probably most folk wouldn't even notice these particular features on their fish but I just happen to spend a fair amount of time by the tank.

Water parameters are as reliable/steady as always - no ammonia or nitrites, nitrates ~10, PH 6.4, etc.

Presumably, in both cases (missing tail portions and white dot/bent spine), given that this is having no adverse effect on behaviour, I leave "as is" and just keep an eye on the situation?

I had a bronze cory with a curved spine. Generally it seemed to prosper for a long time. It was smaller than the other cories but apart from that was a little buzzsaw. Sadly, I lost it in a general tank conflagration caused by my hospitalisation a couple of years ago.

As an update on the above posts, Huge Cory lost the remainder of his tail about a week later - I witnessed another pygmy cory bite it and another section out of the other cory's tail. Since then, he has remained hyperactive but was lying completely on his side on a plant leaf one day last week and still hadn't moved several hours later or when I lifted the tank lid, so I went to remove him, only for him to torpedo round the tank and resume his hyperactive swimming up and down the glass; I can only assume that a fortnight of hyperactive swimming had worn him out and he was taking a much-needed rest. However, this morning, he was struggling to go up to the tank surface to take a gulp of air, and was rolling around quite a bit when trying to swim. On return from work, he was doing the same, and also looked very washed-out like Dead Cory looked in the couple of days before he died 2.5 wks ago, so I immediately moved him into the small container and noticed that his tail stump is slightly red, so have treated him with a generic medication (Aquarium Munster Medi Mor, which I bought from my LFS as it doesn't sell the eSHa products which folk here generally recommend) which treats fin rot among other conditions.

Quarter Tail Cory seems to be fine although has had to adapt his swimming slightly, mainly to set off from a sitting down position. 

Two cories, both long and thin, and one of which is the biting-tail culprit, seem to be fine - I'll refrain from naming them Healthy Cory 1 and Healthy Cory 2, to prevent tempting fate.

Bent Spine Cory with the white fleck which comes and goes (usually but not always on a Thurs, but which disappears on a Fri - before the Sat'dy water change, so not connected with building nitrates) has been doing fine but was lying on a plant leaf, completely on his back/upside down, yesterday afternoon; after waiting a while, when I went to remove him, he torpedoed off round the tank and seems to be fine although the white fleck is evident. I'm wondering if he may have found that lying on his back actually provided some relief for his bent and presumably weighty spine.

My questions therefore are:
* do I set up the QT this evening and move Huge Cory into that to treat him?  [I don't want to leave him in the small container with only 4L water and no filter or heater for too long.  I can move one of the two filters across into the QT and have a spare heater.  I think he may struggle to reach the water surface to get his gasp of air until he's feeling a bit better.]
* do I move Bent Spine Cory and possibly even Quarter Tail Cory in with him to treat them as a precaution (or maybe even all 5 cories), and give him some company?
AND/OR
* do I treat the entire mainstream tank (which houses the x-ray tetras and harlequin rasboras)?

NB. Water readings have continued to remain exactly the same as per original post, even with the addition of the harlequins recently.

Thanks in advance for your help.

Thanks, Sue - treatment has been underway for a few days now. It has definitely helped Huge Cory's struggling and rolling which have stopped but I fear he may have finrot as the posterior part of his dorsal fin looked eroded and his body looked shorter yesterday evening. However, I'm determined to give him and the others the best possible chance of pulling through this - and he seems pleased to have company now in the QT, having spent the first night on his own.

When treatment eventually ceases, ought I to change the filter media or would a quick rinse in some to-be-discarded water from the main tank suffice? [I had moved the second filter in its entirety into the QT.]

Thanks, Sue - no shrimps/snails, so I'll proceed accordingly.

Fish in the quarantine tank have been very lethargic and not eating - possibly the lack of stimulation from other tankmates or the lack of substrate to forage in isn't helping, or perhaps it's the effect of the medication - but I'm hoping that the rest that they're having may ultimately help them to heal.

The water change is a good idea, just before the med dose.

Red around the vent and gills suggests something else going on. The red vent vent is an indicator of an internal infection which could be bacterial, viral or protozoan, especially if you also suspect dropsy. I've never noticed other fish coming down with dropsy when the odd one of mine has had it; it's always been just the one affected fish, then a couple of years later another one. Red gills can be anything from ammonia in the water to maybe even just the body shape pulling the gill covers more open than usual so you can see inside better.

If I had fish with the symptoms of fcmf's fish, eSHa 2000 is the med I would use. It is what I used when my previous betta had what I thought was finrot - bits missing from the edge of his tail. [In the end I decided it was just him, the edges of his tail became ragged shortly after I got him, they never got better and they never got worse in 23 months]

Sorry for your loss fcmf.
Hope the other cory makes it

Today is the third/final day of dosing the QT with eSHa 2000.  I'm beginning to think that lack of social stimulation is now the bigger issue despite a still tail-less fish and a fish with a wound and two bends in its spine ie dorsal fins are erect, eyes look bright, etc, and the sitting on the bottom seems to be more of a boredom issue, so perhaps re-integration into the main tank soon is wise.

When moving the filter back into the established tank as its second filter, will the good colonies of bacteria still be present/able to reactivate, if the filter had been supporting 17 fish in the main tank and then suddenly only ~2 in the QT for the past 10 days? Ought I to go ahead with the original plan (ie a good squeeze of the filter media in old tank water from the main tank, just as I did after Huge Cory's death alongside the 50% water change in the QT) OR, given that Huge Cory may have been suffering from something more complex, ought I to discard this old media (or would we assume that it's fine, given that the remaining fish in the QT have not been adversely affected) and put in new filter media, with a little bit of mature media from filter 1?

When they were both in the main tank, the bacteria population would have been split between the two filters in proportion to the size of their media. And don't forget that a good portion of the bacteria will be on the glass, substrate and all the decor/plants so the filters combined would have had less than 100% of the total.

If there is no disease in the QT filter, put it back in the main tank. The remaining filter, and the surfaces, will have increased their bacteria numbers after the QT filter was removed and the 'spare' bacteria in the QT filter will still be there, though they may have become dormant by now. If you move the filter back with the fish, all that will happen is that you'll have a few too many bacteria so the 'spare' ones will go dormant and eventually die.
Should you decide not to put the filter back, adding 2 fish to the main tank will cause no problems as it is safe to remove up a third of the media when seeding a new tank. But yes, put some new media in the QT filter, and move over some media from the other filter, placing it first in the direction of water flow. This seeds the new filter and over a few weeks the bacteria will spread out into both filters in proportion to their size. Just like us, bacteria are constantly dying and being replaced by new ones.