I have seen different opinions regarding using Lego or hamster cage tunnels (colored connecting type tunnels) in aquariums. Have you used either of these things in your tank? Success or failure? Risky?
They're made of plastic and nearly all the plastics you're likely to come across are aquarium safe anyway so there's no problems there.
For me, personally, the question is do these type of gimmicky things look good in an aquarium. I prefer a more natural look so i'm afraid having a multi coloured lego brick structure in my tank would never happen.
But we all have different tastes so whatever floats your boat.
I maintain a small tank for the kiddos classroom so I was looking for colorful decor. Many of the items I have bought in the past from the decor section of LFS have actually peeled paint on me and had to be removed from the tank.
In that case then if it's for the kids classroom a few lego structures and whatnot would be ideal to capture their attention. Lego bricks certainly won't peel. You'll probably have to weigh them down though because of the air pockets trapped in the bricks.
I can't comment specifically on Lego or hamster tunnels, though agreed flotation would likely be an issue. That said your thread title did catch my eye, as I'm clearly not immune to using unusual tank decor myself lol (I designed his house, he made the driveway, the rocks were previously piled up in the middle).
Anyway with the Legos maybe you can have a contest to see which team of kids can make the coolest fish house
I’m not one for artificial scapes but just like aquascaping, it’s as cool as you decide to make it, and at the end of the day, it’s YOUR tank, so you will be happy with what you like to see. This isn’t my tank, it’s one I saw online a few years ago but thought it would be a perfect example here.