I use a 400 gph pond pump from Lowe's for flow in my 55g river tank. It has 3/8" (maybe 1/2"?) threaded outlet and inlet that are handy for this type of thing. When I do a water change I hook a hose up to the outlet and pump, and it takes a few minutes to drain the water through a hose to my sump. When I'm ready to fill I hook the loose end of the hose up to my faucet and run the water right back through the pump (I'm on a well, so chlorine isn't an issue.) It works nicely and cost about $50 when I bought it on sale.
I may have missed it, but any elevation difference between your tank and tub is going to make a major difference in the flow of the pump. Just a consideration. If you want to use a pump that requires submersion (for heat control, usually) you can always put it in a bucket of water in the tub. It will simply overfill the bucket and remain cool.