Do you have a link to this list?

11. Not matching the fish to the water chemistry. For example, somebody with soft water buying common livebearers that prefer hard or even brackish water.

But in my book, the mistake about impulse buying should probably be number 1.

I don’t like # 2 as a mistake. I always wash my filter material under the tap. The “beneficial” bacteria is important but more so in tanks that are not heavily planted. In heavily planted tanks, they compete with the plants. I mostly use my filters for water movement and mechanical filtration but I understand that if you are new to the hobby you probably don’t have the type tanks that I do. In that case wiping out the bacteria in your filter is a bad thing.

I do think that for Livebearers not having real plants in the tank is a huge mistake. In my opinion, the benefit that they provide for livebearers is immeasurable. If you want healthy livebearers, especially Goodeids, you really need to provide heavily planted tanks. Not only do they provide cover for the fish but they provide areas for fry and adults to graze on.

The list is clearly aimed at beginners. I can think of exceptions for items 1-8 for an experienced aquarist. For instance, say you have an opportunity to acquire a fish that is not easy to find, even if you aren't necessarily looking for it. It qualifies as an impulse buy but any of us here would be able to properly accommodate the fish even if they had to spend a couple days in a bucket with airstone and heater first.

I'm not sure what happened to the link... but it's a pretty easy one to find .
Some great points here!

All livebearers are not easy Mel, but the common poeciliids at a LFS are. It is and is not a true statement. For a typical general aquarist who is in their first year or two, livebearers are indeed "just add water" fish. As we learn about the diverse ranks of livebearers that only serious fish keepers ever see, we realize that they do not all fit that description. The challenges that we often see in livebearers are not likely to ever be seen by a casual aquarist. Instead they will go on thinking that livebearers are very easy to care for and breed.

Here's my "Two Cents" which may or may not have been worth giving.

Of course my reference will be toward Fancy Swordtails & Platies.

It's easy to buy fish which others have worked hard to develop and fix as a strain, place them in a tank a let fry be born.

It's hard to develop a strain such as Albino Platy or Lyretail Platy through a cross with Swordtail. I know this because I have done it!

Darrell E. Mefford (former Fancy Xiphophorus Specialist)

Darrel, I know that you are an accomplished breeder but for most of us just having healthy fish that reproduce and give us enough of each species to pass along to others is good enough. Although I can admire and even envy the success that people like you enjoy, I never expect to reach that level of expertise. I do not consider myself a fancy fish breeder but more a caretaker for species of wild type livebearers. I focus on goodeids but really like the wild Xiphophorus species too. I have seen and admired, at very close range, some of your creations while acting as a runner at the ALA conventions auctions. What a thrill to see such great looking fish. The fancy strains are definitely what keeps generating new interest in livebearers among non-specialists.