This is probably a stupid question, but can you use cotton balls topped with clay pellets for a hydro medium?

No..............

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And don't be letting my son leave Advanced Nutrients empties all over the place....that's a dead giveaway!!

fair enough. never grown using hydro so pretty much clueless on it. just though id ask

No worries.......I personally use 3" rockwool cubes placed in a nursery pot half full of hydroton, then I cover it with more hydroton. I get these products at a hydro shop 2 hours from my house or online. The easiest thing to obtain to grow hydro in would be vermiculite or perlite, which you can buy anywhere that has gardening stuff. Read the growfaQ (if it's working today) and it should instruct you in the proper use of the perlite/vermiculite mediums.......I have never used them.
B

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And don't be letting my son leave Advanced Nutrients empties all over the place....that's a dead giveaway!!

thanks man, ive only ever been growing in soil. heard you can grow quicker in hydro setups so i was having a look here to see

try coco, very good hydro medium.