If its an 04 series Fluval (104, 204, 304, or 404) you can adjust the flow with the gray lever to 50% according to the directions. My question would be are they not comming to the surface at all or do the danios and tetras still feed on top? If the filter hasn't changed where they eat it might just be a matter of them getting used to the new flow of the tank. I have a 50g tank with a Fluval 404 and my neon tetras and harlequin raspborra don't seem to mind the flow. I can tell there is current along the bottom of my tank by how my peppered corys swim, but I haven't seen anything that would lead me to believe they don't like the flow of water either.