Planted Tank Guru
Join Date: Jan 2010
Location: Contra Costa CA
Tablet fertilizers are good.
Mondo Grass is not an under water plant. It will die slowly, not grow.
There are not many ferns that are OK under water, and the few that are should not be planted in the substrate. Java Fern and Bolbitis.
If you can post pictures of the mystery plants, and the umbrella plant perhaps someone can ID them.
Many house plants will barely survive under water for a while, and there are stores that will sell them as aquarium plants. :-(
As long as the sand is not too deep there should not be problems with anaerobic pockets forming. Under 2" is good for most grades of sand.