how does sickness's start?

Most diseases will only be present if you have introduced new fish/inverts/plants recently(recently is within 2 months). However, some diseases are caused by inbalances or defeciencs in the diet (but aren't contagious) and injurys via tankmates/decor (this disease is called fungus) so it is safe to say that you can get a disease at any time if you aren't maintaining your aquarium(s) right.

dorkfish said:

so it is safe to say that you can get a disease at any time if you aren't maintaining your aquarium(s) right.

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And sometimes even if you are.


One myth that has been perpetuated in the hobby is that Ich(a parasite) is always present in your tank. The reason people used to think this, is that the parasites can live on the fish with no signs of its presence for long periods of time. If a fish's immune system is comprimised in anyway, the ich will take over and possibly kill it. This is just an example of how a well established tank can have problems with no new introductions of critters.

Fish can get sick in several ways. One of the most common is stress from improper water changes. The other is the introduction of new fish without qt ing it first. Last but not least, cross contamination.