fish compatibility

Question
Hi
i have a 40 litre tank with 18 fish
12 neon tetras and
6 harlequins
neons used to shoal together initially when i had six only and now with twelve of them they wander aimlessly
well point is both neons and harlequins are healthy

i also have some plants cabomba
(it keeps uprooting how many times i ever plant it)
hemianthus some twigs my friend was happy to give its growing but really slowly

now my questions r
1.can i add a betta splendens fish (i have him in a bowl he s happy but i feel sorry for him)
without him harming my other fish
and
2. can i add some red cherry shrimps?
i am having lots of algae growing al over so will they help control the algae?
if yes how many should i bring and will they be compatible with my fish including the betta?
hoping for an answer.,....

Answer
Sachin,
You are grossly over populated. Too many fish in that small of a tank. I cannot recommend adding any fish to that tank, but I suggest you either get a larger tank or rehome some of the fish. They are over crowded and they don't know what to do. DO NOT add a Betta to that tank. DO NOT add the shrimp. DO NOT add any more fish.