Done a lot of classes for this idea of mine of Space D&D. Thought I would go ahead and put forth one of the races for the game-

The Fishmen of Planet Aquaria-

This is a race of sentient fish humanoids. They are capable of breathing underwater and swimming at accelerated speeds. Because they use gills to breathe with they must wear a gill tank anytime they are out of the water. They must also immerse themselves in water for at least 10 minutes every 24 hours or they start losing 1 CON per hour, reaching zero CON means they are dead. CON points take 1 day per point lost to recover. Using a water suit will make it possible to avoid the immersion requirement but its water must be changed at least once every 7 days.

To make a Fishman character you must have a minimum of 9 CON. 

Fishmen are able to swim at a rate of 15”.

They can breathe water.

All fire damage does an additional +2 damage per die.

All cold, frost, and ice attacks are reduced by 50%.

Fishmen are limited to level 6 as Spacemen, Level 8 as Scientist, they like all races are unlimited as the Explorer class.

 Please note that this is not my picture it is something I found on the internet. If the owners of this copyright wish me to take it down I will.