After reading your other post about your pump flooding the system in two minutes, I would think it safe to drill a small hole at the low spot in the reservoir to drain it completely after the flood cycle.

here is the problem with a picture,

what tube connector should i use to connect it to that new hole ?

{{gwi:998254}}

I see. The problem is in the large connector! I assumed your nutrient was draining directly into a bottom reservoir and a simple hole without fittings or tubing attached would work fine. If your reservoir is directly below, remove that large fitting, run a circular bead of silicone on underside around the hole to control the dripping while draining and there you go.

If that fitting is necessary part of your system, perhaps you could modify it by cutting a groove to the center to allow the standing water to drain?

the groove idea will definitely help, i'm gonna do that and try to not break the connector ^^

but still i will have a bit a solution left anyway, so will this be a problem when the root start to deep in ?

i'm guess that the root will die in there, how about putting a thin layer of gravels to support the roots ?

Think I would look for a different container that doesn't have all those low spots next time!

I once made a conical bottom to a barrel by CAREFULLY heating it with a torch and GENTLY pressing the upside-down barrel onto a piece of standing pipe rested on the bottom-center...low spot for settling of fish waste. I suppose any plastic container could be made with a low spot for drainage this way.