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I am trying to make a mini green house out of an aquarium. I need to get the seeds high to the light. Right now they are in plastic bags.
But when they get bigger and ready to leave their plastic bags I want to put them in the aquarium. I am trying to think up what soil I will need to fill up the bottom so the bottom won't break. I have some floral foam blocks for flower arrangements that use those then put the soil medium on top.

I'm not sure if floral blocks hold up in the long term. I'm not sure they won't but you'll want to be sure because taking the whole thing apart to replace them would be a real pain. I'd also worry about them turning into sponges and keeping the soil too wet.

In terms of other options, off the top of my head, you could also try putting down a few layers of old glass jars to reduce weight (plastic may also work, but I'd be worried about it being too light and starting to float up when the soil's being watered)

I would use large Styrofoam flat pieces to raise the height. On top of the Styrofoam I would place a weed screen and then put the soil on top of that. Styrofoam will reduce the weight, weed screen will allow the soil to remain in place and will allow water drainage. I use this method to reduce the weight of large outside pots.

Just to make it clear - your seedlings are in pots and pots are in bags, right?
If so why would you need soil in the aquarium?
To put seedlings close to the light you can use regular styrofoam or empty plastic pots flipped upside down. This way you could adjust the distance to the light for different seedlings if needed

My seeds are still in the germination phase. I try to get ahead of things on things that might need thinking thru. And get peoples advice.
Right now they are on the arrangement foam still in plastic bags so they are near to the light.