A pool does not necessarily decrease the value of a house, it instead decreases the number of potential buyers which could force you to sell the home for less because you need to get out of it due to financial restraints or a poor market. If an appraiser came to a house that had a pool, mind you it has to been in working order or something close to it, it would not decrease the property value. It would almost always add value, but only a small percentage of what you put in to have the pool installed. There's a difference between property value and the price you sell it for. My house appraised at $238,000 but I only paid $170,000. It is still worth that higher amount, but due to market constraints and a financial problem with the seller I simply lucked out.
A fish tank falls under the same type of improvement, but again it wouldn't decrease the value per an appraiser, but it would decrease the market of potential buyers down even more. Now at closing some people might demand the tank be removed, but again that really has nothing to do with the actual value. You simply decreased the "pool" of buyers.
Now even worse is when you look at the state of today's housing market. Now you have to figure in the fact that your home likely lost value, not because of changes made to it, but because the demand is low and so many other factors. Let's put it this way, it's complicated.
I would simply warn you that a specialty addition like this won't really drop your value, but expect to have a much harder time selling it

...in essence it likely drops the price, but not the value. Oh economics, isn't it fun!