The reason I never made any money selling fish is that I would never upsell and the tanks I set up were for the fish not the fish keepers. Like this tank belowTrue, but you have to keep in context of what and who these sites speak too and warn: beginners.
They get recommended and sold bunch of "hardy beginner' fish.
Those "hardy beginner" fish are probably one of the hardest ones for beginner to keep alive these days. Because they are heavily inbreed and carrying 5 deadly diseases each.
They aren't treated at all at shop or anywhere because they are cheap and as shop: if you lose 6 out of 8 you still make 100% profit margin on selling 2 of them. That's how business works.
Plus you are going to sell bunch of medications to customer trying to stop the great die off. That's called product upselling.
Then you are going to sell replacement fish to those same customers. So as a business: it is not really in your interest to treat those fish and sell them healthy. If anything that would be bad business model.
As a beginner: You can't really tell sick fish unless it's far gone. And you definitely can't tell bunch of parasites and deadly diseases on fish that looks healthy.
And that's what the sites OP is talking about warn beginners about.
Hardy beginner fish are definitely not hardy beginner fish anymore.
The $20-30 dollar ram or angel is 10x hardier then platy from my LFSs. Because they are mostly in good shape and treated in shops for monetary reasons.
Having said that: experienced fish keeper would probably be able to cure and save most of the platies from the shop that aren't too far gone already.
7 Year old Tank
I thought I would share this with you. This is Nancy's Tank I set it up for her 7 years ago. It is 250 Liters. It has two internal filters. A bank of four Florescent light tubes that run 10 hours per day. The fish are all between 5 and 7 years old. This tank has never had: Any Chemicals...
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