A:  There are three reasons drinking water in boat tanks can taste “off.” First, the water was of poor quality in the first place. This is unlikely in urban areas but more common in remote rural places where the water may be biologically sterile and safe to drink but contains elements that give it poor flavour. Second, something in the tank or system (usually bacteria or mould) may be causing the problem. This is serious because these contaminants pose a health risk. Third, an “off” flavour can come from the materials used in the tank or system. Often identified as “chemical” or “plastic,” these flavours usually do not pose a health risk, but are unpleasant at best.

In the first case, flush the system and replace with good tap water. This will usually clear up the problem.

In the case of bacteria or mould, more aggressive action is in order. The regime you outline is more than thorough and should purify the system. A good bleach rinse is usually enough, with a caveat for aluminum tanks: leaving a concentrated solution of bleach in the tanks will produce a whitish deposit on the aluminum. Any municipal water system has chlorination treatment, so all tap water has a very low concentration of chlorine bleach in it. This level poses no apparent threat to the aluminum tank material, so any treatment of aluminum tanks that involves bleach should end with a thorough flush-out with tap water. Many commercial treatments are also available for aluminum tanks from recreational marine and RV suppliers.

However, for all tanks, flushing the system won’t remove solids. There’s no use sterilizing the system if there is still crud in it; dead bacteria are just as foul as the living variety! And it will only encourage the problem to return. First, take a look inside your tank. If you see sludge or slime, remove it before the treatment. Water tanks should have access ports large enough for you to get an arm in. In a really serious case it may be necessary to replace the hoses as well.

When flushing the system, run all taps long enough to clear the treatment (i.e. bleach) out of the water lines. Continue to pump out and flush with clean water until the bleach and bad flavours are gone.

All this will be to no avail if the tank and plumbing lines are made of materials that impart flavour to the water. The standard for tanks is food grade high-density polyethylene, although aluminum is also used. These materials should not impart significant flavours, but fibreglass tanks tend to impart a styrene smell, especially when new.

Plumbing should be entirely done with proper potable water hose or pipe, pumps and fittings.