Problem in My Community Fish Tank | 2 Feet Aquarium Setup

This is my own community fish Tank. It is a 2 feet Aquarium Setup done 10 months ago with very small size of fishes and now you can see how they have grown in size.

Recently I have tried to change in some looks of my tank by Adding black sand and lava Rock. I have bought this sand from Kurla Fish Market but, it proved to be a wrong idea for my tank set up, as my water was not getting cleaned as it is supposed to be with these many fishes.

Suggestion : Only add the sand when you have small fishes or less number fishes or using canister filer or sump attached to your tank to clean the water.

Tank : B : 1.5F, H : 1.5F, L : 2F.

Fishes are :

Veil Tail koi fish : 2
Gold fish : 3
Zebra fish : 3
Pleco (Sucker fish) : 2
Scar Angel fish : 2
Ghost fish : 2
Tiger Shark (Iridescent shark fish) : 2

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