Gujustud,
I use 802s with Quickfilter attachments filled with foam in my 42g hex and in the 72g bowfront. The two in the bowfront are on full throttle, one pointing across the back of the tank, the other pointing so the water hits the glass 1/2 way to the middle of the tank.
In the 42g, I have adjusted the single 802's water flow down to about half, otherwise there'd be water all over the dining room
To adjust water flow, I find it best to remove the PH and break it down partway. If it's dirty, break it down all the way and clean it while you're at it
Anyway, what you are looking for is the red lever on the outside of the PH which moves the green thing inside the PH over the outlet. If you try to move the red lever while the PH's still running, you'll never be able to do it due to the force of the water. If you try to move the red lever while the PH is off, but not broken down, you won't be able to see how much of the outlet you have covered up for the next time you want to adjust the flow.
Also, I would probably use only one 802 in your 38g, or as someone else suggested, two 402s.