GRP Water Tanks - Fouling Prevention?

[caiusflatulanus]

Advice gratefully received!

Background: I have an Aussie-built GRP 30' Clansman (circa 1976). The built-in grp freshwater tank occupies the forward half of the V berth and extends forward under the chain locker. There is a beam-wise grp baffle and the tank holds 100 litres. When I first bought the boat in 2003 the water tasted disgusting. I found out why when I prized-open the antique inspection port! The balooning, multi-coloured growths were impressive..... I flushed everything out with bleach and replaced all outgoing hose with taint-free drinking water hose, replaced the taps and installed an inline charcoal filter between the tank and the taps. All of that, however, seems like cure rather than prevention. The inside of the tank appears to have been painted with some sort of cream/off white heavy-duty paint - flowcoat? Bilge paint?

Question: What would any of you do to these tanks to prevent growth and fouling in the future?

(Random musings: 1. Drayton Tank in the UK produces a solvent-free epoxy paint which is used for lining fibreglass water tanks in some UK schools i.e. meets strict health regs. 2. After installing a filter for me on land, between mains and house, a plumber remarked I should cover the clear acrylic mesh-holder because algae requires light to grow. Good quality solid fibreglass is translucent.....)

[DeepFrz]

As you don't know what the tank is lined with, coating with solvent free epoxy seems like a good thing, although a big job. Every bit to the interior has to be cleaned and sanded in preparation for the application of the epoxy. Probably several coats will be needed. Even after that you will need to clean thoroughly on a regular basis. Filtering the water on it's way in to the tank will also help.

[Sonosailor]

(I realize I'm responding to an old post, but to improve the knowledge base...)

Light: I would agree that blocking off light will decrease the growth inside fresh water systems, but it will not get rid of it. There will always be a slime growth forming in the tanks and water pipes. I would coat the exterior of the tank with a solid dark gelcoat or marine paint to decrease the light. If you do suspect some solvent or flavour leaching from the existing interior coating, then use a dark solid colour when you apply that no/low-solvent epoxy instead of the exterior opaque coating. Follow the instructions to the letter - it should say to build up the thickness with many thin coats rather than to apply thickly, I imagine.

Turnover: Regular turnover should help to solve the problem of foul tastes and odours, but if you do not have turnover, you should increase flushing and maintenance.

Disinfection: I believe you will find some good threads on the use of chlorine on this site. Even chlorination will not get rid of the minor biofilm. It will, however, keep it in check.

Just my opinions - feel free to dispute or add yours.