plant problem, please help

I am growing in a 20 gallon tank with two air stones and pumps, I have a metal halide 250w lamp for 4 plants of which the tallest is about 24 inches. I have kept the light on the entire time, and am about to switch to 12/12. The roots are dangling out of the bottom of the pots submerged into water, I've been told to cover the tank so that light does not get to the roots and I'm still wondering what eles I need to do. I also have some red stems and an algae problem. some wilting leaves also. I'm going to get a 250w hps light soon, to run both the metal halide and the hps at the same time during flowering, any help would be appreciated, thanks.

roots dont like water cover them up NOW!!!!..THEYRE ALSO STREYCHING SO URE GONNA NEED MORE LIGHT ASAP...

you need to not let the light hit the water cuz you will get mold and fungus, and like the other guy said you probably going to need more light

you need more light i wouldnt suggest mh for veg i would suggest u get a sunlight cfl system something type of HO CFL like 6 54 watters, lower the light and dont flood them the reason u see you plant stretching is due to the light height, bring that shit much lower i suggest 4-8 inches from the top of ur babies

and give them some nitrogen thats what causing the yellowing of the edges and tips of ur leaves, the lack of nitrogen that is

nitrogen like kushy said..nitrogen she wants...and lower ure light....is theyre a fan blowing on them?they need it if not....but i dont know why theyre selective problems for select plants...arr they the same strain?

Why are your roots out like that?

Roots + light = big problems and root rot.. Fix the issue so u dont run in to deeper problems.

rofl i didnt even notice that. yea dude get that tank taped up or some shit your gonna get root rot if u dont already have it