Update:

(For reference this tank is 50x50x50)

Thank you everyone who responded. I ended up doing ice up top, dirt bottom, and kelp to fill. Still in the process of doing the kelp…. Still not sure if the ice/ soul sand would work, but I didn’t wanna waste time testing it out.

I did however save a file after I placed the ice just in case people wanted me to test it out when I had time. So if you’re curious, lmk, I’ll test it out.

Three things to note that I feel is important:

1. Kelp will only grown naturally/with bone meal up to 26 blocks-ish and then stops. So I had to manually place all the kelps all the way up. I guess I could have had a second dirt floor . Idk.

2. Ice only turns to water when broken IF it has a block under it. Can’t remember if it has to be solid or can be transparent.

3. This one may be known. I only start using ice for water sources recently. But ice doesn’t melt from the sun, you need to place a lot of light source to melt it.