You are using horizontal water nipples from day 1 and they are actually using them and getting water?  Is that your only water source?  I tried my chicks on the horizontal nipples at 2 weeks and they don’t seem strong enough yet to use them.  The horizontal nipples I got from a friend so not sure what brand they are or if that matters.  I also ordered some RentACoop brand horizontal nipples but haven’t tried them yet to see if there is a difference.  I now keep the bucket with horizontal nipples in the brooder with a quart jar waterer to be sure they are getting enough water.  When I manually push and water comes out, the chicks 

 come running to drink 

but I haven’t seen them do it on their own.