Clone, you tank is probably still cycling so as to speak....high ammonia or nitrite might be the reason why your fishes die (CO2 poisoning as well as pH crash might be another culprit as you have not mention adding baking soda to buffer your water?)...not the filter flowrate that killed your fishes but it does stress them even more as they cannot find a place to rest due to the strong current.
Do readup more as suggested...
Here are a few good sites to start.
The krib
CO2 injection
Plant Physiology by Taiz and Zeiger
Bookmarks