Beginner fish tips!

Context: this was in response to someone asking for tips on what fish to get for their first tank, or if we had any general advice. As I do, I got wordy.

First, buy a test kit (you'll want it eventually, believe me). Test your tap water's pH. It's a lot easier to work with what you've got than to try to "chase pH" and try adjusting it — especially as a first-time fish owner.

If it's 7+, then go with livebearers, like guppies, mollies, or platys! They like slightly alkaline water, so they'll be more likely to thrive for you. Ideally, have a decent-sized tank for them (I'd say 20+ gallons — it feels counterintuitive, but genuinely, bigger tanks are SUBSTANTIALLY easier to keep stable anyway, because there's more water keeping everything stable). Then, and I can't stress the importance of this enough: buy 1 male and 3 females (or 2 males + 6 females, or at LEAST buy them in that 1:3 ratio). They will multiply faster than you expect, no matter how fast you expect it, and if there are too many males, they can not only show aggression towards each other, but they can literally pester the females for sex until they die. (Men.) Get lots of plants (I suggest Marketplace!) for the fry to hide in — too bare a tank, and none will survive long enough to stop being food for mama and papa.

If it's <7, then you've got a much wider range of tropical fish that you can go with! Here are some suggestions:

• Stay away from cichlids as a whole. They're not beginner fish, generally.
• Stay away from barbs (except maybe cherry barbs); they can be semi-aggressive and are imo not a beginner fish. (Note: this includes the GloFish!)
• Neon tetras are ubiquitous... But they're so damned inbred in captivity at the moment that they fairly often have health issues and as a result aren't nearly as hardy as people think they are. Avoid em, imo; if you really want the look, get black neons instead! They're even cooler-looking, IMO.
• Zebra danios are a great beginner fish. They're pretty hardy, can survive some beginner's faults, and look cool when they are schooling/shoaling. You'll need to buy at least 6, ideally more, though — which will be the case for pretty much all of my suggestions here. (For most of them, including danios, sexing them isn't the easiest thing and it's not super necessary; with 6 you'll likely have a male and a female, who will likely breed... and then eat their eggs/fry, most of the time. You won't really need to worry about it, for the most part.)
• Alternately, go to your local fish store (ideally not PetSmart or similar, IF your town has an alternative in town or nearby... you might pay a little more, but the fish will have been kept in higher standards (almost guaranteed), and are less likely to have disease) and check out what interesting-looking tetras they have! Even my local PetSmart probably has 7 or 8 cool tetra species at any given time. Buy at least 6 of the same species, and ideally a few more; less than that, and they won't feel secure and will be stressed.
• Bettas are tricky to have in community tanks, and having just one fish — even one with personality! — will probably feel a little underwhelming as a newbie. I'd make that your second tank, if you want one.
• Mystery snails are... Hit and miss, imo. They need more specialized care than you'd think. You'll end up with "pest" snails if you go with live plants, anyway, more than likely (and you should go with live plants, they naturally help keep your parameters in check!) so that snail itch will be snail scratched.
• Be careful when choosing to mix shrimp with fish, or small fish with large fish. Fish are opportunistic to a fault, and anything that can fit in their mouth WILL become food, more than likely.
• Cory catfish (of all kinds), kuhlii loaches, and bristlenose plecos are fantastic and help clean the bottom of your tank for you, but: cories should ideally be kept in groups of 6 as well, kuhlii loaches need 4-6 minimum AND hide during the day so they're not the most exciting fish in a tank for a newbie, and bristlenose plecos... surprise, you can get only 1-2 and they're fine! Any of these three can be in a tank with live-bearers like guppies as well, unless your water is particularly alkaline.
• One good approach to stocking a tank is to have high swimmers, midway swimmers, and bottom feeders; that way, everyone is happy, all areas of the tank have movement, and they're largely not disturbing each other. Danios tend to keep to the upper-middle, most tetras to the middle, and then cories/loaches/plecos are bottom, as a rule of thumb. But be wary of OVERstocking, since then you'll need to do constant water changes to avoid deaths — another reason to get a larger tank to start!

Regardless of what you go with to stock:

• Please consider live plants! They're a bit of an investment (though less of one if you get them primarily on Marketplace, and buy less than what you want to eventually have and let them grow on their own), but they're SO helpful. The end result of a proper nitrogen cycle is a buildup of nitrates, which are toxic to fish at high ppm — plants use nitrates as fertilizer, so they naturally keep your nitrates down and will mean less-frequent water changes are completely fine. They also break up sightlines for the fish, which will reduce or even eliminate any aggression.
• Make sure you cycle your tank before buying fish, and then buy the fish one species at a time so that your cycle can adjust to the new bioload — give it like a week between species.
• If you're doing live plants, the special plant substrate IS genuinely worth it, imo, though some people swear by just having regular ol' potting soil. Either way, cap it with an inch or two of sand. (Trust me, don't make the same mistake I did. Go with two inches. It WILL be cloudy af otherwise.)
• Over filtration is your friend. I have three filters on my main tank, providing roughly twice the filtration support I need. This means that a) if one of them breaks, I'm fine; b) my tank can be slightly overstocked without an issue, and c) if (when) I want to start a new tank, I don't need to wait and cycle it from scratch — I can put in one of my filters from this tank and it'll be basically insta-cycled!