yes, DD1, very good test idea. my thought is you'll probably be o.k. on the water quality, who knows, maybe the stuff they emit will prevent or inhibit aquatic weed growth but not hurt the fish. you might find something new here...........contributing forum members would like the royalties in small unmarked bills please
since they are established mature trees and provide your only shade (based on that first pic) i can see why'd you want to keep them. i'm not sure what species those are, but having mature root systems might not be as big a problem for a liner now as planting young ones and putting a liner over young root systems that will be expanding rapidly.... if you follow me.
euc's are very tough and have their place. one of the reasons they were originally imported back in the 1800's. they provide good wind breaks and shade in Godforsaken areas all over the west. just dont put buildings near them as rad alluded to.
if you ultimately decide to take em down, they also make great firewood....looks like you could get alot of cords out of those
