I don't know if I would set the seedlings down inside it. You want very good airflow around seedlings to prevent damping off diseases and to promote good leaf transpiration.
I use a small fan, set on low, a several feet away from my seedling flats. After the seedlings put in their first set of true leaves I put the fan on them for a little while every day. Not blasting them with it, but situating it so the plants just barely move. This is to help strengthen the stems. Now some people believe in doing this and some don't. After a few decades of starting my own seedlings I believe it does strengthen and slightly thicken plant stems.
If they were down inside an aquarium, they wouldn't get much air circulation and disease and improper transpiration would be my biggest concerns.
However, do you like rooting cuttings? If so, and aquarium is excellent for that purpose. Everything from easily rooted flowers and herbs to woody plants like fig tree cuttings root very well in an aquarium. You can either set your perlite/rooting medium filled pots down in or as I've done when rooting impatiens, coleus and figs, I put a 6" layer of perlite in the bottom, dampen it, and stick the cuttings directly in that. Put the lid on and check daily, opening the lid for a several minutes a day to promote healthy air exchange and to check moisture, and then put the lid back on. I still have a couple 10 gal. aquariums in the greenhouse I use from time to time.