thanks everyone for the comments. that substrate is just an ordinary black sand 1-2 mm. i know i have too many but i have these goldfish for more than one year with fancy pebbles and plastic plants. i changed the water 50% weekly but i'm thinking to do it more frequently.
this morning i wake up and find some hygrophila corimbosa leaves are floating, i guess they nibble it last night. no uprooted plants yet, keep my fingers crossed
for the foreground i'm planning to use some cryptocorine parvula or sagitaria subulata, but i'm a little worry because the front tank is their feeding ground for 1 year. it's my habbit to feed them at the front with sinking pellets so i can watch them eat, they tend to screw with the sand searching for food, suck it with their mouth then blow it away.