White spot etc

[fatguppy]

Maybe a silly question but where does white spot/fungus etc come from? I understand about quarantining new fish etc and our readings are fine - ammonia/nitrite on 0, nitrates around 10 (7ish in tap water) but althgouh everything is looking good, we have white spot.

Is there anything you can do to prevent it or is it just "one of those things"?

Thanks

Jane xx