No! this is not a good idea, I'm not sure about recent changes in the lego corporation (I gave up after they rollled out a whole bunch of new lines in 6 months, then followed that with bionicals, who knew they could make one toy in 6 colors and sell two of each to each kid, legos are different k'nex is the new budding engineer toy) anyways as recenttly as 2001 the standard lego blocks get slimey if they go in water, the boat shells that they rolled out in the early 90's can handle the water quite well but if you hold them in your hands you can tell that its a different plastic. In the fish tank the traditional legos would eventually break down because believe it or not some plastics are not waterproof. Who knows what they leech when they do, best to avoid the whole mess if you ask me.
HTH
Opcn