First Time Hydro System User

I am new to Hydro Growing. My system is a Flood and drain system. The timer is set to flood my top tray every four hours, just long enough to fill it up. Which is 3 minutes. It works really good, but one thing I am haveing trouble with, and that is that my pots want to float off the bottom of the tray and flip on their sides. I have Mickey Moused them to get them to stay down. By laying screw drivers and such acrossed them. I really do not like this idea as they can fall off. One of my pots flipped over today. Luckily I was there when this happened and saved it.

I am thinking I should have put some gravel in the bottom of the pots for weight and to help drain. But I do not want to do this at this point now.

Here are a couple photos so you can see what I am working with.

Thanks for any suggestions you guys can give me.

if you have other pots that came with the system you can put them back. you can fill the whole tray with hydroton/gravel. they don't look so big you cant transplant them.