Recent Lego bricks have been more eco-friendly and thus are biodegradeable, I'm not sure how many issues those may provide.
While looking up about putting a Tamiya 1/350 scale Tirpitz model in an aquarium I mostly find things for plastics leeching chemicals and stuff.
Things to keep in mind, I never did add the Tirpitz model because I was never able to become certain one way or the other of its safety to the fish.
Lego bricks are made primarily of ABS (acrylonitrile butadiene styrene). ABS is not biodegradeable and will not leach anything into the water. Lego have recently done tests on a more eco-friendly friendly sugar based material, I think this is what you may be referring too, but this too is not biodegradeable, and nor will it ever replace the bog standard ABS bricks.
All plastics are safe for your fish tank. The only ones I wouldn't trust are plastics with flame retardants added in at the extrusion stage. Also, if you're using plastics that have been in contact with other products, for example a plastic tub that may have originally contained detergent. This is where you need to be careful because the detergent can leach into the plastic tub, and subsequently leach said detergent into the aquarium water column, we're talking miniscule quantities, but enough to cause problems in a fish tank if you used that tub, unwashed, in your filtration for example.
So basically, new plastics are 100% safe, unless containing flame retardants. And used plastics that have been used for storing something in a previous life, are best to be avoided unless you take the time to soak and wash the hell out of them.
PS. There are biodegradeable plastics out there and they would turn to mush in your tank and make a right mess, but biodegradeable plastics tend to be packaging such as plastic carrier bags or bin liners. Such bags that tend to find their way to landfill where biodegradeable properties are advantageous for obvious reasons.