My first fishtank was an excellent example of how not to set up a community aquarium. Granted, I was a small child relying on advice from my "expert" father (who once put ice into an angelfish tank because it was supposedly too hot outside). I loved those fish dearly and thinking about them fills me with nostalgia. However, the entire thing was an absolute disaster. Here are the mistakes I made that I hope you won't...
1. Overstocking - I had a 40 gallon tank with several angelfish, two pictus cats, three gold barbs, three black widow tetras, three goldfish, two tinfoil barbs, and two platies. This is a nightmarish situation in terms of bioload, especially considering my heated tank housed three goldfish whose metabolisms were on overdrive because they aren't actually tropical fish. We used the goldfish (feeder goldfish) for cycling but, having named the trio, I couldn't bear to return them.
2. Not researching fish species before buying- Impulse buying is so common (and so dangerous). Here are some things I didn't know about the fish I excitedly introduced to my tank. First, angelfish are territorial and establish a pecking order within the tank. In a tank that small and crowded, it is easy for the less assertive angelfish to endure frequent bullying with nowhere to hide. Second, my fish were destined to outgrow their home. Tinfoil barbs do not belong in a 40 gallon tank EVER unless it is temporarily being used to house juveniles. They can grow to be 14 inches long and are very active swimmers (keeping them in a small tank is cruel). Third, some fish require shoals to thrive (of varying numbers). I would recommend keeping black skirt tetras in a group of around six. Pictus cats often behave skittishly when alone and are also happier in a small group, requiring a very large tank (I don't think 40 gallons is sufficient for even one). Fourth, not all species require the same temperature and water parameters. My tank was probably a very comfortable temperature for the angelfish but the others were not as pleased.
3. Change your water (often)- I definitely did not change my water frequently enough. This was an especially egregious error considering my massive bioload. Some people are scared to change their water too frequently in fear of shocking their fish. Just make sure the water is treated and not too hot or cold. You will do a lot more damage to your fish letting them drown in their own ammonia.
4. Sand is fantastic substrate- Yes it's not ideal for a gravel vacuum. However, it also doesn't get dirty as quickly as gravel (no pockets for food and waste to become trapped in). Aside from being very natural looking, there are some fish species that actually prefer having a substrate they can sift around.
5. Wash anything you put into the tank- I thought that a quick rinse of decor in the sink was sufficient. I didn't even bother to wash the gravel. Soaking decor and using a strainer on gravel is ideal. Sand can also be cleaned by soaking and sifting.
*Eventually I grew mature enough to do some research and realize the error of my ways. I re-homed my tinfoil barbs, some of my angels, and all goldfish, upgrading to a 55 gallon tank (still not enough but the largest my parents would allow). Now, I have a much larger setup that I like to think of as an improved version of my first community. For example, I have six angelfish instead of the unsustainable nine I initially kept. However, I increased the numbers of pictus cats to six and blackshirt tetras to thirteen in order to make them feel more secure. There are no platies (which a large pictus cat might try to eat), goldfish, or tinfoil barbs in sight. My disaster of an aquarium inspired me to later create a beautiful and thriving community. Hopefully my disaster will help you do the same.