Water purification

[Daydream believer]

My fresh water has been in the tank since end of March. I drained it as best as I could at the end of last season. I have not used the boat this year for various reasons & although it had a dose of Aquatabs when I filled it the water still has an eggy smell. I have drained it in the last couple of days put in more aqua tabs & refilled. The water now smells of chlorine & eggs. I do not drink the water as direct, but use it for making coffee, cooking & washing.
Looking at various options ( it normally gets a lot of use & I do not think that it is dirty itself- same for the water heater) I want something that is effective & will kill any possible germs, clean the tank. I am thinking of buying some--- "Aqua sol water treatment"
Has anybody tried this? Is it effective or is there something better that they can recommend? I would expect to drain everything & refill with the required treatment. I would rather just do this once though as there is a lot of water to shift
Thanks for comments

 

[geem]

Do you use a carbon filter? My experience is that a carbon filter removes any residual taste after cleaning. We drink from our tanks and the taste is perfect.
If you use one of the homebrew sterilising products and leave it for 24hrs, these work well. Make sure you run some of the treated water through the pipes as these can get pretty grubby, and allow that to stand for 24hrs as well. Drain and refill and use carbon filtration.
We use 10" filter housings. A 1micron filter in front of a 10" carbon block filter. The 1 micron filter becomes your barometer of water tank condition and protects the carbon filter from debris thus making last a little longer.
If that 1micron filter is getting grubby, time to access the tank for a proper clean

 

[Minchsailor]

Have you thought of using hydrogen peroxide?. Powerful oxidizing agent, and virtually tasteless.

 

[thinwater]

You did not mention cleaning the tank. The first step is scrubbing out any growth, because bleach/chlorine cannot penetrate the growth. Get a long-handled brush and a hose.

The sanitizing procedure has been formalized by both WHO and ANSI, and is presented in the below link. Use plain (not scented) bleach--no need for special chemicals (Aquatabs are very good for maintenance, probably the best, based on Practical Sailor testing).
Sanitizing Procedure

The egg smell is from bacteria in the water converting sulfate in tap water to H2S. Carbon won't do much to reduce H2S, but it is very efficient with chlorine (the carbon is actually consumed by the chlorine).

As for H2O2, EPA and WHO do not recommend it because if the dose is high enough to works, it is toxic to people. There are no protocols for treating drinking water with H2O2 (if you have one, please post it). Bleach is what works. But clean the tank first.