Hi,
Daphnias and gammarus need a lot of oxygen.
For gammarus, you can perform that with a bubbler. Don't forget to put some dead leaves in their tank (for hide and food).
For daphnias, it's a bit more complex since bubbles in their shell can make them float.
Aquatic live and active plants like Egeria, with a good light, may be a solution.
In any case, these species don't thrive well in too warm water.