Try running the Rena with empty media baskets. If the flow improves you've got the baskets packed to tight. If the flow stays the same there is another problem.
75 Gallon--Rainbow Fish Tank/Amano Shrimp
29 Gallon--Celestial Pearl Danios/Yellow Shrimp
20 Gallon--Ember Tetras/Orange Shrimp
10 Gallon--Chili Rasboras/Cherry Red Shrimp