There are companies that make soil that doesn't float in water for water plants. Most garden centers and stores should carry that kind of soil. It's labeled for water plants.

Hi
This is for the aquarium part of the setup. I'm using
top soil with about a third compost. Floating is not a problem Disolved organics,
In the past I've used everything from sand to peat to cat litter and gravel . All work and all have limitations
Since I'm trying the Lace plants again and I have such a large amount of water decided to go with the enriched substate particularly since I have direct sun
i use straigth cat litter on plants with floating leaves.
or sometimes peat and gravel.
i suspect those soils marketed for aquatic plants are overpriced cat litter anyway lol.
I was going to skip submerged plants but what the heck
have always wanted to try this and will probably never have the opportunity again.
The ultimate planted tank!! Just requires a lot of patience and mess lol. gary

It's the peat that floats, if there's no peat in the mix, it shouldn't float (unless there's also perlite, of course).

Lucy
if you use peat boil it first and it will sink. Also removes some of the excess tannins. This also works with vermiculite but not topsoil will cause it to separate.
I'm not having a problem with floating just excess nutients. Looks like i took a leak in it .lol
it will clear when the plants start to grow and of course it doesn't help to have a waterfall and 1200 gallons of circulation lol. Will just take time.
gary

Gary have you tried any of the Cryptocoryne species before? They would be interesting submerged and marginal plants, especially if you can get them to bloom.

Hi
i bought 35 different kinds lol. Aroids are one of my favorite families. IME Crypts do better emermed with a lot of mud but it's possible to keep them sub. for long periods of time. The flowers are typical aroid but rather small
and of course only last one day. It's easy to get viable seed from them but not worth the trouble. My favorite s among the aroids are Alocasia. No sub but do love the water.
I have 3 flower stalks emerging on the Lace plant but suspect they will abort from transplant.
Trying some Glossostigma in the bog. have never been able to grow it sub. Have never been able to get it to flower either. incredibly beautiful when well grown.
gary

Gary - wish I'd known that years ago when I was dumping piles of earth into our pool for the frogs to winter in!

lucy
I've always been amazed at how much more difficult it is to grow plants in a tank as opposed to a pool.Submerged ,10x more difficult than emerged. obviously the more water the easier it becomes.
I had almost decided to skip subs in this setup due to the difficulty but since I have such a large area??
To me, one of the great joys of gardening in enclosed spaces is the experimenting.
gary

I have never grown Cryptocoryne before but I am certainly intrigued by them. I have been reading Aroids by Deni Brown and found the section on Cryptocorynes very interesting.

Mike
Crypts are interesting and even better they're easy lol
I think they would grow in a closet with a bit of water lol. You can grow them easily in a 10 gallon tank which makes setup and lighting easy. They flower easily if allowed to emerge. there are many varieties and most cost less than two bucks.
The super family of Aroids is unbelieveable in diversity.
gary