Others have already covered what might have gone wrong (plant or dead bodies decomposing creasing excess pollution, overfeeding - leftover food decomposing, live sand, etc.) I was trying to think of any other possibilities given you are new and may not consider some of the basics. Was your old tank under direct sunlight? While direct sunlight is fine when you are cycling the tank, it can kill them (like boiling them alive) if the shrimps are inside. I would go for artificial light or just be really careful in making sure there's enough indirect sunlight (enough as in enough to make algae).
And if you have to feed the shrimp because you are having trouble growing algae, I would avoid any food you cannot remove (i.e. spirulina powder) as the uneaten part will just decompose in water. I offer a shrimp pellet once a month and then remove it after a few hours to make sure it won't pollute water. Best luck to your guys!
One last clarification, they don't eat the algae that mustafa sends. The invisible algae they eat grow on those macro algae or mossballs, so what you get is not food, but a decoration/surface for food to grow.
What did I do wrong
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