Pool pump motors aren't fed with a GFI breaker, but spa's are, why is that? Do I need to feed an underground aquarium with a GFI breaker? If so, does it have to be fed from the 125a sub-panel, to a 60 amp sub-panel that will control the motors, or just 1 GFI breaker for each pump motor?

I think it is an oversimplification to say you GFCI spas and not pools.
This started with the "skid pack" spa that was assembled in a factory and seldom inspected by anyone. NFPA required that to be GFCI. Field assembled units were not required to be GFCI. As code cycles marched on more and more got the GFCI requirement. Now it is only 3 phase or very large field assembled spas that are not GFCI. Pools are also increasingly required to be GFCI. If it has a cord and plug, it has to be GFCI. I think the same will be true of your fish tank. In 2005 this is article 682.