wow,thanks for all the responses;
first, iv"e never been on the net asking for help so please bear with me ,
the den was a garage27x27.with a door into the house .as for the critter containment,the top 6"of the pool is above water and is concave and tiled.humidity i was concerned about, wasnt sure if a fan would suck it out or not.
never crossed my mind about fish jumping out ,have to study on that one .
i do have quite a cross ventilation just from normal air flow .
i was planning on pouring concrete and tiling over that.question arises , would it be harder to keep water clear if i put rocks ,dime size and smaller on bottom. i dont want to spend all my time cleaning rocks .
well again thanks again for the come back,myold brain is working over time now.