I'm not sure how crystal clear you will get your water using an undergravel filter, as how useful it remains long-term will depend on your maintenance schedule, and how well you clean out the gravel. Clarity will also depend on the amount and type of substrate used.
My suggestion would be to ditch the pool pump, and use something designed for an indoor aquarium, with replaceable and changeable media. Trying to blaze a trail and do something from left field using unusual methods and equipment is probably left to the more experienced.
I'm not sure what sharks you consider adding, but some may not allow more than themselves to remain in the tank, so planning to have stock that will most likely get along will be best unless you have another tank set up to use for a plan B.
You may want to read about cycling too. before you think about getting any fish.