stupid question
 

I have an aqua tech 40 gallon filter(for 38 gallon tank), but it sucks water up at the bottom with a tube that goes up strait through the top and into 2 net type things to trap whatever gets in the tank, wont this suck up and eventually kill my smaller fish that i want to get, like a neon tetra?

Hopefully not. Strong, healthy, fish tend to avoid the pick up tubes of filters for fear of being eaten. Fish are kinda smart that if they feel the force of water going in a different direction than they want to go, that they will generally swim away from that spot unless weak or ill. Since your filter has a screen like bottom even if a healthy fish does happen to get drawn towards the pick up tube, if healthy, they should be able to overpower the actual siphon and swim free. Once a fish does get drawn to the pick up tube it is not actually stuck to it as water is now being drawn from another area of the tube so the suction is actually lessened where the fish would be "stuck".