Essentials:

They are not just kobolds, but a water kobolds (or a fishbold, as i will address to them) - a branch of kobolds invented by me (but I may not be the first). These kobolds have two forms - land and water. In their land form, they are a normal kobolds, except that they smell strangely fishy and have a rather massive tail, but as soon as they enter the water, they transform into their water form. Their scales change color to be more suitable for underwater landscapes, their scales also become slippery like fish, their body grows fins and gills on their neck, allowing them to be agile in the water and not need to float for breath .

Details:

Fishbolds have big tails that are often stronger than the kobold himself, as it is the primary fin that allows them to be agile in the water. Sometimes it can even be longer then kobolds height.

In fishy form they can smell, but they can't differentiate between different smells. It's all the same. But they can feel sharpness of the smell.

Their disadvantages:

Their problems start when even a drop of water falls on them (they will transform partially, it can be even one scale) and they are seen that way - in most cases they become pray.
Because their blood is very important in potions it is quite expensive.
In a certain condition it gives the user a water form with fins and gills (if there is a tail, then a large fin will appear on it, but it will not add the tail strength like kobolds have, because fishbolds were born with it and have gained strength by themselves through their lives). But this potion works in doses and before the end of the effect, the user will not be able to regain his normal form, even if he goes on land. Unlike fishbolds, the user of the potion will not be able to breathe with his lungs until the end of the effect. Potion user's blood can't be used for potion making, it will transform back as soon as it will be taken from the body.

Please leave a comment if there is something needed to be added for bigger picture. It will help me a lot!