One neighbor said that if there is no soil washing out of the leak, then no worries apart from loosing water. It likely wont self seal though, perhaps lessen over time. You could try those polymer sealant products.

Dont bother with duckweed control with chemicals, it will be a constant battle. It is there because there are excess nutrients in your water. Plant some desirable vegetation to absorb the excess and add some fish that eat the stuff to keep it at bay. Cut back on the feeding.

I have Koi though there may be some better choices. I tried putting some DW in while feeding my perch, and the Koi ignored it until the fish food was gone. Then it was down the hatch! (I have an ample supply in a garden pond in need of work)

Good luck!