Dear Bohuš Kubala the literature is plenty of desinfection protocols. You must addapt them to your conditions. There're some key factors that you must to consider. (1) Explant: select a explant source with the lowest posible contamination. (2) Desinfection agent(s): depending of contamination of explant, you should select a suitable substance (or physical method); in some cases combining some of them it's necessary. (3) Desinfection treatment: it's important to reach a point in which your protocol remove as many as contaminant you can without killing the explant. Also important, don't subestimated none step, all of them are critical. Good luck.

Here's a basic protocol I've used in the past for plant samples

5-20% bleach (need to optimize for your tissue) with 0.5% detergent (triton or tween or even dish soap), incubate with gentle agitation 2-10 minutes (you'll need to optimize this step too). Then rise 5 times with sterile water.

You can try this protocol:

Rinsed under running tap water.

Use Bavistin (0.1-1%), tween-20, then again rinsed under tap water for half an hour.

LAF: 0.1 % HgCl2 for 30 sec, 70% ethanol for 30-40 sec and rinsed 3-4 times with autoclaved distilled water (5 mins each).

As you are trying to sterilise(surface) plants growing in water, maybe try in a smaller "pond" to grow them in water added chlorine

Solution for sterilisation. And also try to have shoots growing above the water level.

Hi Bohuš Kubala

Surface sterilization is a tricky process in tissue culture; if HgCl2 is allowed in your place, optimize HgCl2 (0.1%) treatment from 3 - 6 min, Followed by 3-5 times sterile distilled water wash under laminar air flow conditions. If you find any fungal contamination, use Bavastine (0.2-1 %) 10-20min. You have to optimize both Bavastine and HgCl2. If the HgCl2 treatment is more means, the explant will die.

Good luck