There are a lot of reasons. Sort of in order of importance....
1. new hobbyists are buying a lot of fish. Every time you buy a fish, you're running a risk, and new tanks are acquiring their populations one fish at a time because they're not 'strong' enough to have a bunch at once---and because quarantine tanks are small.
2. Water quality New hobbyists often don't test beyond nitrate and ammonia,and get fish before they realize they need to test more than that. Water with a low alkalinity is a frequent problem, water with unstable salinity---no autotopoff. The philosophy of buying fish before equipment leads to problems. Trying to operate with no skimmer, no refractometer, no alkalinity test, and while overstocking... are all problems down this track.
3. Buying sick fish. Very many tender-hearted new hobbyists will see some shy, weak-seeming fish and want to help him by introducing him to the nice new tank where they plan to make him healthy. What's going on in reality: the fish is carrying parasites. Buying healthy fish is a learning curve, and it frequently means an active, pushy, and obnoxious fish is the healthy one. Look at belly (should be plump-ish, no white poop)...look at gill covers (tissue should not be showing underneath and the gills should not be flaring). Look at skin, fins should not be ragged. Watch the fish's swimming: is it energetic? Ask the owner to demonstrate the fish is eating aggressively. And provide that food for him.
4. Buying from the cheapest source. ...not always the best deal. Ask around.
5. Sharing quarantine equipment with your DT. No. Don't. Not nets, not sponges, not thermometer probes: if it's been in qt, it needs NOT to be in your DT.
If you're doing everything right, you will very rarely see ich or other disease at all, and if it shows it will show up in quarantine, not in your DT. Give your quarantine its full 4 weeks (some go 6) and be serious about it. If you're seeing ich develop in fish you're buying---buy from some other source. Clean source matters! and if a seller's tanks are infested, they're not a clean source. I have every sympathy for the difficult life of fish store owners and plead with people to buy local if you possibly can, but if you have gotten infested fish from a source, don't go right back for another.
1. new hobbyists are buying a lot of fish. Every time you buy a fish, you're running a risk, and new tanks are acquiring their populations one fish at a time because they're not 'strong' enough to have a bunch at once---and because quarantine tanks are small.
2. Water quality New hobbyists often don't test beyond nitrate and ammonia,and get fish before they realize they need to test more than that. Water with a low alkalinity is a frequent problem, water with unstable salinity---no autotopoff. The philosophy of buying fish before equipment leads to problems. Trying to operate with no skimmer, no refractometer, no alkalinity test, and while overstocking... are all problems down this track.
3. Buying sick fish. Very many tender-hearted new hobbyists will see some shy, weak-seeming fish and want to help him by introducing him to the nice new tank where they plan to make him healthy. What's going on in reality: the fish is carrying parasites. Buying healthy fish is a learning curve, and it frequently means an active, pushy, and obnoxious fish is the healthy one. Look at belly (should be plump-ish, no white poop)...look at gill covers (tissue should not be showing underneath and the gills should not be flaring). Look at skin, fins should not be ragged. Watch the fish's swimming: is it energetic? Ask the owner to demonstrate the fish is eating aggressively. And provide that food for him.
4. Buying from the cheapest source. ...not always the best deal. Ask around.
5. Sharing quarantine equipment with your DT. No. Don't. Not nets, not sponges, not thermometer probes: if it's been in qt, it needs NOT to be in your DT.
If you're doing everything right, you will very rarely see ich or other disease at all, and if it shows it will show up in quarantine, not in your DT. Give your quarantine its full 4 weeks (some go 6) and be serious about it. If you're seeing ich develop in fish you're buying---buy from some other source. Clean source matters! and if a seller's tanks are infested, they're not a clean source. I have every sympathy for the difficult life of fish store owners and plead with people to buy local if you possibly can, but if you have gotten infested fish from a source, don't go right back for another.