What happens in these cases of "disappearing grout” is that the calcium level in the water is too low. Because fiberglass pools provide no source of calcium (unlike concrete pools), the grout around the tiles is slowly eaten away – as the grout is the only source of calcium available.
For this reason, it is imperative that fiberglass pool owners maintain a higher level of calcium in the water, with the optimum level being 350 parts per million. Compare that to the standard 250 ppm (which is what any pool supply store will recommend as the correct level) and it’s easy to see how this mistake can be made.
For this reason, it is also very important that fiberglass pool owners frequently test the level of calcium in the water. In addition to there being no source of calcium in the pool, variations in water temperature and pH levels can cause the calcium levels to fluctuate up and down. So if you own a fiberglass pool that includes tile grout, I recommend that you test the calcium level at least every other day – and every day, if possible. This way, you’ll avoid the shock of jumping into your pool one day and discovering that all of the tile grout has disappeared.