BIG POTS!!!!!!!!!!!!!!!
Trying to grow organically is a lot easier with big pots ! I think you're going to struggle in flower in five gallon pots. If you're going to use that small of a pot, you need to make sure you grow small auto strains.Aeration is key! Do you self a favor and spend a little bit of money and get some volcanic pumice. Don't get just any pumice. It needs to be high in silica and LOW on heavy metals. It will last you many years and also contribute to the health of your plants..
Make your own ferments with Korean natural farming or KNF for short! J, fermented fruit juice, is very easy to make and you can store it for quite long while. That will be very beneficial to you in flower, with that being the most likely place you're going to run into problems with nutrient issues.
I'm gonna be honest with you, give up the thought process of a water only grow. @hecno Is about the only one here that comes closest to that idea, but he also adds some of his special fish pond water!
He also has media cooking at all times. When he finishes a grow he takes all that media and Re-amends it and then let it sit and cook.
In a few days I'm starting a new grow and doing a lot of new things............... Automated watering and under canopy lighting. It will be in all autos sponsored by AutoSeeds . It will be four different strains in seven 15 gallon pots In 3 tents. I purchased all new media from BAS, Build A Soil, here in the US. Is probably possible that that media could go full term with no problems with a water only, but I'm not gonna do that. My organic program is more of a living soil organic grow. I'll be adding top dressings of compost, earthworm castings and probably in this grow also a living mulch along with straw. I'm still going to be using all my different ferments and weekly use of beneficial bacteria. By continuing to use these amendments, I'm not exhausting the nutrients that much in the media. It makes for a quicker turn around for either taking all the media out and cooking it or a continuing grow in the same pot.
Now I'll admit my program is not for the first timer growing organic. I only started growing cannabis 5 years ago but I've always grown organically all my life, and I'm an old fart ! I've never grown in containers until I started growing cannabis. I'm still learning what organic techniques I can use growing cannabis in containers.
Since you are in the EU, you definitely have access to a very interesting bit of products that are extremely easy to grow very nice plants. There are several growers here that use the full program from Bio Tabs. They get just simply outstanding results from using it! We can't get the full program here in the US due to regulations. If we could, I definitely would try it out it ran a side by side.
It's definitely something that I would look into in your case. It's too early and I haven't finished my first cup of coffee to remember who here uses it , but search here for Bio Tabs to see which growers use it.
I can't stress to you enough about pot size! I've done side by side test with the exact same media and the exact same amendments with two different pot sizes. Not only did the bigger pot grow bigger girls, the buds were also bigger and better formed..