Need To Get To The Bottom Of What's Going On Here

Last night I put my barbs back in the main tank as I wanted to remove the red eye tetras that seem to have some nasty illness or injury going on with their heads. I've put them in the hospital tank with  a product called bacteria control. Then this morning I find my little harlequin swimming round unco ordinated in the big tank with the injury to its underside. This is now in my last available tank my little 13 litre. It looks like it is dying now. What on earth could be wrong now?

i am no expert but it looks like a open wound and seeing as you didnt see it last night could something have attacked it , what alse was with it in the tank.

is it possible the ruby shark has attacked it , i have a ruby shark in my tank and if its in one of its moods then it is quite happy to evict any of my fish from its territory even my snakeskin gourami,s which are alot bigger than it.the crazy swimming could of been as a result of injury or stress.

i know exactly how you feel cazzy a few week ago i was in the same situation to you i was returning home from work every day to a dead fish and i was down and fed up with it blaming it all on myself , i think i went my first 9 days of fish keeping to 9 deaths in my tank but now i am doing better and things seem to be settling in and now   i am back at the stage of enjoying fishkeeping again , i think iam a few weeks ahead of you having bought my first fish a few weeks ahead , so hopefully you will get all your problems solved soon and can start to enjoy it to.

was the barbs all healed up nicely btw ?

Oh, Cazzy....I'm so sorry that you are having a miserable time of fishkeeping at the moment....but it will be only a moment out of all the lovely, relaxing times that you will eventually get with your little fishy friends.
It seems to happen that at the beginning when you are trying to get a settled, balanced tank that you can get a blip with some fish. I'm sure it probably happens to most of us, I know that I had some ups and downs in the early days. At the risk of jinxing things, my tank has been troublefree for quite a while now, so don't give up.

I'm no expert, but to me, your little fish looks like it had an injury more than an illness.....maybe a bite? Sue will have more idea though, and I'm sure, will give you invaluable advice.
Hope things start to improve, we're all rooting for you

Oxygen will only be a problem if you don't have enough surface movement. Gas exchange takes place at the water-air interface, oxygen goes into the water and carbon dioxide (made by the fish respiring) goes out. [Before anyone with heavily planted tanks makes a comment - if you have a lot of plants, they will use carbon dioxide during the day and you may even have to add it. And plants make oxygen during the day]
If the water was still, you would be limited in the amount of fish you could have by the 1 inch of fish per 12 square inches of surface rule. But the filter does more than remove ammonia, nitrite and particles from the water; it also churns the water up. It circulates oxygen rich water from the surface to the bottom and it brings oxygen depleted water to the surface where it can absorb more. The main cause of lack of oxygen is the filter not being positioned properly. The outflow from the filter should be positioned so that the surface of the water ripples.

The harlequin does look like it was injured. I too would blame the shark. The red eyes would also be on my list of culprits if you hadn't moved them.