mine are everywhere,lol...medium flow is probably good for them...were your water parameters good? 0s? salinity and temp and ph good? there are so many things to cause it...check for any parasites like zoa pox, spiders, nudis, sundials? they are small and can hide EXTREMELY well...i have had some of mine die off lately, and realize my salinty and ph have gone a bit too much i believe.