Corydoras Pale, Fin Loss, Loss Of Balance

No meds until you've tested the water. A lot of your beneficial bacteria live in the gravel, so don't over clean it.

Hi Sue, thanks for the advice.

Firstly, ive tested the water - ammonia 0, ph 6.5-7 nitrites (shows the lowest on the colour chart)
so all looking good.
I've not put any meds in yet...now what to do? ive now spotted another cory thats pale and the rear fin is corroded, he appears to be losing his balance also...

The snails, yes ive tried the lettuce leaf method and didnt really work for me either.  The jar sounds interesting though, thanks.  There are literally HUNDREDS. The tanks crawling with them.  I bought 2 assassin snails some years back and they are still in there, but the population of the 'other' snails has increased. Theyve eaten most of the plants now too. The tank is full of white shells where the snails have died at some point...not good. I try not to clean all of the gravel - i do one side when i clean out, and the other the next time i clean out.  The problem is, i wont kill the snails and once i can get them out, i never know where to put them!!

i wont kill the snails and once i can get them out, i never know where to put them!!

May I ask why you won't kill them? If you do not wage war you'll never get them under control.  From what I have read, an explosion of snails indicates an abundance of food in the tank.  If snails have moved on from algae to plants then it sounds more like the food stock is running out and they are taking steps to keep going.

Under such circumstances I would be removing the source of food entirely ie: all the remaining plants.  If they can't get enough grub their numbers will decrease.  Decor, remove it and soak them in very hot water for a few minutes before a little scrub with a dedicated toothbrush (or similar).  The hot water soak is to kill any egg sacks.  Glass cleaning too, to scrape off those egg sacks (just done this on my own 180ltr last night.

Is it possible that the snail population is making the fish unhealthy?  I don't know if this is possible, but it does sound like a very overcrowded environment.

If anyone else has any suggestions I'm sure they will add.

I'm learning about snails too at the moment, as I had some invading my tank when I planted it. Keeping a watch on population which is sub-ten in number, but they are laying egg sacks.  I'd hate to have a population like yours, sounds awful. 

I understand perfectly.

It may take a while to get on top of it.  As there are so many snails, if they do start to die off because of lack of food, their bodies will sink to the substrate and start to rot quite quickly.  This in turn will produce more waste, making it very important to keep on top of substrate cleaning, water changes and water testing.

As ColinB has said, there is a lot of goodness in the substrate, so too much cleaning has a detrimental effect.  Looking to remove obvious waste, such as discarded food, dead plants and fish/snail poo.

Water testing is absolutely vital of course.

In your previous report you gave:-

pH: 6.5 - 7
Ammonia: 0
NitrIte: (lowest colour) <- should be 0ppm

What about NitrAte?  Although NitrAte isn't harmful in smaller quantities, it can be a problem if it gets too high.

Also, what type of water test kit do you use, liquid or strips? (Liquids are more accurate)

If you test the water again, be sure to do each of pH, Ammonia, NitrIte and NirAte and post it here as you go.  It will help us all get an idea of what's happening. Test your normal tap water too to get a gauge on any differences between tap and tank.  Keep a glass of tap water at room temperature for 24 hours, and test the pH again after that.  This allows the gases to dissipate and give you a better pH reading. This doesn't change for me, but others it does, just depends on your own water supply.

If your pH gets too low it can affect the Nitrogen cycle completely, though I have only had that problem when setting up my 64ltr tank.  At a pH of 6.5 it is getting quite low.  If your tap water is higher, then at least we know how to help the balance a bit.

Again, as per ColinB's suggestion, no meds until water is fully tested.
Don't mix meds, use only one type at a time when trying to treat different things.
Make sure you remove the Carbon filter/bag from your filter before dosing meds, and leave it out.
After the run of treatment, put a Carbon filter/bag back in for a few days to help remove old meds, then take it out and throw it away.
Then start your next medication.

Water changes should be done after doing any cleaning to allow more contamination to be removed, dose medication after a water change. Water testing should of course be done first.

Phew, that's a lot to take in and I think I've got the basics down.  Others will chime in if there's any finer points that need talking about.

Anyway, first thing is to get the water tested fully so we can see what's going on.

No problem.  Any of the dechloronator brands will do the job, I've used a few different ones over the last year and am currently on Nutrafin AquaPlus.