New fish keep dying? It is almost certainly the nitrogen cycle, not bad luck.

The most common fishkeeping mistake is buying the tank and the fish on the same day, and very few pet stores will tell you not to. Here's the actual mechanism. Fish waste and uneaten food break down into ammonia, which is toxic. In an established tank one group of bacteria converts ammonia into nitrite, which is also toxic, and a second group converts nitrite into nitrate, which is comparatively harmless and gets removed by water changes and plants. Those bacterial colonies take weeks to establish. A brand new tank has none of them, so ammonia spikes with nothing to process it, then nitrite does the same, and the fish are burned and suffocated by their own water. That's new tank syndrome. It's why the first batch dies and, if you replace them right away, why the second batch dies too. The fix is cycling the tank before any fish go in it. Fishless cycling means adding an ammonia source, either pure ammonia or a pinch of fish food left to rot, then testing until you watch ammonia rise and fall, nitrite rise and fall, and nitrate climb while the other two sit at zero. That sequence is the colonies coming online. Takes anywhere from about two weeks to two months. Bottled bacteria helps some, and a used filter cartridge or a cup of substrate from a friend's established tank helps a lot. You need a liquid test kit to do any of this. The API freshwater master kit is the standard one. Paper strips aren't accurate enough at the low end where it actually matters, and "the water looks clear" tells you nothing, because ammonia is invisible. Last thing, since it comes up constantly: a betta in a bowl is subject to exactly the same chemistry as everything else. The small volume makes it worse, not better, because there's less water to dilute anything. A cycled, filtered, heated 5 gallon is the usual minimum and the fish behaves like a completely different animal in one.