Koi Fry Advice

Okay, so I have managed to rear 6 fry from an egg batch, I have them in a small 12L aquarium, bit I wanted to know, is this little pump making too much of a current for them? Will it stress/kill them? I want to keep as many as possible so ideally all 6...

I put a bit of jaycloth over the output trying to slow the flow but it doesn't seem to have had any effect