I'm new to MyFishTank.Net; I found the site by doing a search on Google for "legos in fish tanks".
My lady and I started a 50-gallon tank in the last month, using Legos as the decor. Lego made several underwater sets back in the mid-to-late '90s called the Aquazone series, and have a current-year set called Alpha Team; these sets offer numerous underwater vehicles and bases.
Our aquarium holds an Aquazone Deep Sea Base and two Search Subs (yellow and black blocks with transparent blue and green windows). The Base is set upon two blue baseplates, and the plates match perfectly with the two-tone blue gravel floor of the tank.
To ensure everything was non-toxic and "cleansed", we placed the Lego sets in the tank, filled the tank 50%, turned them upside-down (to allow air trapped beneath the plates to exit), then set them down into the gravel. After adding the rest of the water, we let our Emperor 400 Biowheel run for an entire week, testing the water every day for ph/nitrate/ammonia/hardness ... and everything has worked out just fine.
We now have a comminuty tank with 4 fantail goldfish (Nixon, Flash, Goldfinger, and Fat Bastard), 3 swordtails (Spike, Blaze, and Diamond), 2 siamese algae eaters (Nessie and Bigfoot), and 4 otocinclus (all named Darrell, cuz I'll be damned if we can tell any of them apart). They LOVE swimming through the base and moving the crane on top of it ... and despite fears of sharp edges/toxicity/etc., we have yet to notice a problem. I have a friend who used Legos as tank decor; he still has a clown pleco and an angel which were in his Lego tank from 5-6 years ago.
Our only aquarium problem is brown algae (which is EASILY visible on yellow Legos!), but the otos and sae's should take care of the issue handily.
Bottom line: Legos are fine, provided you set up the tank intelligently; your fish will have as much fun with it as you will!