I own rentals with pools in hot and sunny Arizona, and they are big headaches. The issues with indoor pools are somewhat different, but pools in general are expensive to maintain and insure. I suggest you check with your insurance company before you complete this purchase to determine if they will insure you for the risk. You will need a high dollar liability policy for this risk. At the very least, the pool will have to conform to local and state pool barrier laws for you to be insurable.
Indoor or outdoor, I would never leave a pool unfilled. Empty pools are dangerous and the shell can shift or crack. Tenants are not capable of maintaining a pool, no matter what they tell you. If you go through with this purchase and decide to keep the pool, you should fill the pool and provide professional pool cleaning and chemical service. This will be difficult with an indoor pool because the tenant is likely to be uncomfortable letting strangers in the house to clean the pool.
In my experience in Arizona, where pools are a desirable amenity, you are lucky to recover half the pool operating cost in rent. I would guess you will recover less in the Midwest.
The better option may be to fill in the pool. I'm not sure how this would work with an indoor pool. You may want to price this option before you proceed.