Hi Matthew, and welcome!
Unfortunately, both you and your sister have tanks that are too small for the fish inside them.
Let's start with your sister. There are a lot of myths about goldfish. People think they can keep them in small tanks or bowls and if if they live a few months that was a success. This is just not true. A well kept goldfish will live for years, decades even, and they actually grow to be quite large. A goldfish that lives for a few months and dies small is not a success, it's a tragedy. It means the fish didn't even live to adulthood. Her tank is half the size of yours, and to be quite honest, even your tank is too small to keep goldfish in. They do best in ponds, rather than aquariums, but if a pond just isn't an option, she really should get a tank that is at least twice the size of your current tank, and that would be the bare minimum; larger would be even better. There is a belief that a goldfish will only grow as large as it's tank allows it to, and there is some truth to that, but that's a sure way to kill a goldfish. When a goldfish's environment is too small, it will stunt their growth, but their internal organs still keep growing, and eventually their organs outgrow their body, thus killing the fish. Which is why it's best to put the goldfish in a larger tank while they are still small, instead of waiting until they outgrow their tank. There is no such thing as a small-breed goldfish. When you see a pond with large Koi or large goldfish in it, those aren't some special breed that grows larger than aquarium goldfish, they are the same breed, they're just older.
As for your tank, you have a few fish that need to be in schools but aren't, and you have a space issue with your rainbow shark. The schooling fish are your Tetras. The Neon Tetras and the Glowlight Tetras need to each be in schools of at least 6. Tetras are a schooling fish, but they will only school with fish that look like them, meaning similar size and coloring, and you need at least 6 fish to create a school. Your Glowlight Tetra will not school with Neon Tetras, even if you bring the total of all tetras up to 6. You need 6 of each. Unfortunately, just having 6 Neons and 6 Glowlights would max out your tank. You would not be able to have any other fish in that tank besides those 2 schools of 6. Keeping fish of a schooling species without the numbers for a school is very stressful on the fish, and they may die from that stress. Especially if you only have one, like the Glowlight.
The Platys are not a schooling fish, but they are a social fish for sure. But there is a general rule with platys that you should keep 2 females for every 1 male. Like guppies, platys have a lot of energy, especially the males, and if the ratio isn't right, males could harass females to death. It's recommended to have 2 females for every 1 male to help spread out the male's attention, so no female is singled out. Yes, they will reproduce, but if you don't want them to reproduce, keeping other fish in the tank will solve that problem, as the fry will become food. Or, you could keep platys of all one gender.
Let's talk about this rainbow shark, though. They are such beautiful and neat fish! But they need space. They can grow up to 6 inches long (15 cm) and are very territorial. They like caves, a lot. A happy rainbow shark will have a cave, the area around the cave will be its territory, and it will defend that territory, chasing away any intruders it can see enter its territory from it's vantage point at the cave. A happy rainbow shark won't be seen very often, as it will be in its cave most of the time. Rainbow sharks are loners. If your rainbow shark is actively swimming with the other fish, it's not being social, it's trying to chase them out of his territory. Imagine a rainbow shark is an old hermit that stays in his house, and only comes out to yell at kids to get off his lawn. With rainbow sharks, tank size (liters) is important, but even more important is floor space. They need tanks with a large footprint. Go for a long tank, not a tall tank, and get something that is at least a meter wide, so there is enough floor space that the entire tank isn't its territory. Breaking up sight lines helps, too. So you could put your shark's cave on one side of the tank, then put some tall grass or driftwood in the middle, and the other side of the tank is where it will chase other fish to when they invade his space. I have been wanting to get a rainbow shark myself, but I don't have an appropriate tank for one. Rainbow sharks are a fish you plan a tank around, not a fish you just buy and throw into an existing tank.
I wish I had better news for you.
Sometimes the facts can be hard to hear, though. f you upgrade your tank to something at least a meter wide, you could house your rainbow shark comfortably, as well as increase your tetras up to schooling numbers. Best of luck to you.
