Well, a 22 liter tank isn't the largest one but if it's just 4 of those platies, it should be okay. There are people who live by the 1cm of fish per 1 liter water rule, but that really depends on the type of fish we're talking about if that's needed or not.
When we talk about breeding forms of platies, it also needs to be known whether we speak about the maculatus (Xiphophorus maculatus) or variatus platy (Xiphophorus variatus). There are way more platy species (so, no strain but species) than just these two. But only of these two mentioned, many breeding forms are bred throughout the years.
Variatus platies are more stretched in body shape and those need more width in a tank in comparison to maculatus platies which have a more compact body shape. I also like to mention that depending on the breeding form, adults can become very large or just moderate till even small. Those are points that can be considered whether a certain tank size will be suitable for them or not.
Another aspect is that there are platies that can reproduce massively or just moderately. Not all females will deliver larges batches of fry.
I also like to contradict a remark given by the mod overhere. Yes, if you look at the country of origin, platies come from a tropical country. But that doesn't mean that platies are tropical fish. If one would like to know where most platies come from and then I mean the localities within such a tropical country will determine whether a platy is tropical or subtropical till even cold. A lot of platy populations occur in higher regions (higher altitudes), where the water temperature can range from 12°C - 20°C. Which doesn't make them tropical fish but at least subtropical. But we're so used to the general rule that fish from tropical countries are tropical fish. But such a statement would be completely incorrect!
I myself have collected wild platies from different localities in Mexico. And believe me, a lot were swimming in cold water. While I'm keeping myself already busy with livebearer ever since 1973, I've dived into the deeper matter of livebearers in relation to their origin, behavior, genetics, etc.. As I'm a passionate livebearer keeper and breeder I put work into the world of livebearers which is way larger than most average aquarist can imagine. The socalled big 4 (molly, guppy, swordtail and platy) is just a small group of livebearer species in the whole range of livebearer species.
I also like to mention when you keep platies (goes for swordtails as well) at higher temperatures, they'll be doing just fine but most people also will forget or are just not aware of it that their lifespan will be shortened. Keeping platies at a bit lower till moderate temperature (like room temperature) will make the platies grow slower (but that's good to become stronger and develop a better immune system) than when kept at higher temperatures.
I myself keep platies, swordtails, guppies and a number of other livebearer species at moderate temperatures till some even cold. I'm doing this for decades and they are all healthy, have a longer lifespan and reproduce normal.
Large finned guppies can be kept at moderate till higher temperatures. This has got to do with the fact that the bigger the fins, the thinner the fin tissue will be which will be broken off when kept too cold. Short finned guppies will be just fine at water temperatures from 16°C- higher. Guppies are one of those livebearer species which have a high tolerance in temperature. And they adapt quite fast to other conditions. These are the main reasons why they've been released all over the world.
Than the aggression mentioned in the posters's post... Yes, the smaller the open swimming space (a large tank which is heavily decorated has got also a smaller swimming space), the more dominance or aggression can take place if you've got one or more dominant specimens in there. If more open space is at hand, the dominance or aggression will be less. In case of dominance or aggression the more submissive specimens will try to escape or will hide more in that case. That's normal. The only thing you can do is to provide more hiding spots or get either the submissive or the dominant ones out. Those are the options.
