Fin Rot Or Nipped Fins Cant Tell

Looks like finrot, however the finrot could have been started with nipped fins. What other fish are you keeping it with?

 

check your water quality because the fish is covered in excess mucous and has cloudy eyes and clamped pectoral fins.
Blue sharks are really sensitive to poor water quality and will suffer in water with any level of ammonia or nitrite, and moderate levels of nitrate. They also don't do well in extemely acid or alkaline water. Try to keep the PH between 6.5 & 7.5.
You might also want to look around for a bigger tank for when it gets older. They can get quite big.

 

All you need is dechlorinator (removes chlorine and hard metals from tap water), what does the conditioner you are using as well claim to do? Unless you fancy building a 12ft long indoor pond, i suggest you euthanize the fish or rehome him once you have finished treating him because he certainly looks like an irredescent shark;

http/www.planetcatfish.com/catelog/speci...?species_id=172


Although its hard to say from the pic, it looks like the fish could have an external parasite called Velvet too (there appears to be something wrong with the fishes skin, like it has a dust-like stuff on it?)

 

I think euthanasing it is a bit harsh at this stage. Just hang on to it until it gets too big for the tank. Then either get a bigger tank or find someone who has a bigger tank for it.

The external skin parasite is probably the whitespot, same as the silver dollar has. Just make sure the water is clean and keep treating the tank for spots.
Make sure there is no carbon in the filter otherwise it will absorb the medication and the fish won’t get better.

 

I know it sounds harsh, but thousands of these fish are sold in pet shops every year and most people that want these fish have them already and most public aquariums already have too many of these fish which they have adopted off fishkeepers who cannot house them- the likelihood of finding someone who can look after such a fish properly is very slim, rehoming to a petshop will probably end up with the fish being sold to someone who can look after it little better. So although it may sound harsh, IMHO, euthanizing such a fish could be better than letting it die in an aquarium too small for it (which the vast majority are- how many people do you know with a 12ft by 6ft aquarium or indoor pond?).

 

Finrot will usually cause a redness around the edge of the fins. It's not the best picture, but i don't see any redness.

On that basis, i'm going to disagree Tokis, and i'm going to say it's probably not finrot (yet).

 

Finrot doesn't necessarily cause redness around the fins (if there was red edging it would indicate a septicemia bacterial infection though to the fins), a grey/white edging to the fins is also a sign of finrot. On another thought though advanced whitespot can also cause the fins to rot (apparently this guys fish have been having whitespot recently).