Good news! You are enthusiastic about keeping aquariums.
Bad news! You learn the hard way some times.
But don't blame yourself... It is hard to get accurate information, or any information from people who are ignorant to the hobby of aquarium keeping. I used to work at Petsmart and had to leave after being so frustrated with everyone who worked there. Now I work at a LFS that knows their stuff. So find a local shop by you that takes time to care for their fish.
The biggest thing people don't realize is that patience is key when setting up aquariums. In order to be happy, think about what you would like your aquarium to look like in the future. I usually decide between a community aquarium, a predatory tank, malawi cichlid tank, tanganyika cichlid tank, south american cichlid tank, brackish water tank, etc etc etc... The most important tool in having a healthy aquarium is that gravel vacuum. Draining (while gravel vacuuming) between a third and half of the water out each week will lead to impeccable water quality. The more frequent the water changes, the less harmful nutrients there are in the aquarium. This also means that the new water added in has extremely similar water parameters as the water that is already in your aquarium.
When starting an aquarium, your fishtank is an unsafe environment for fish. The tank lacks nitrifying bacteria that are prolific in established aquariums. This bacteria converts toxic ammonia (fish waste and overfeeding) into nitrites, and eventually less toxic nitrates. In an established (cycled) and healthy aquarium, there will be zero ammonia and zero nitrites present and a slight amount of nitrates. Plants (and algae) use nitrates as food, and they are the only thing that will remove nitrates from your aquarium naturally. This is why it is important to continue doing water changes in an established aquarium, because while nitrates are not extremely harmful, they act as a secret killer, building up over time and slowly taking a toll on your fish.
That is a lot to digest, but that is basically how an aquarium works.
This is what should have happened to your levels...
Once you added a few fish, you should have seen a rise in the ammonia levels.
After a week, do a water change.
Over a week or two, the ammonia levels should have decreased and a rise in nitrite levels should be seen.
This means that the population of bacteria that converts ammonia has risen to appropriate levels to deal with the ammonia.
Continue water change schedule.
After another week or two, nitrite levels should also decrease and nitrate levels should rise.
This tells us that the population of the second type of nitrifying bacteria has risen to appropriate levels to deal with the nitrite.
Continue water change schedule for rest of life of tank.
You may add fish during this process, but slowly. And preferably fish that can tolerate higher levels of ammonia. Just stay away from delicate and expensive fish. Expect to lose every fish you buy, and that mentality will definitely hold you back!
So now that you know how everything works, hopefully you can make yours work!
So. Now. On to the fish.
You have an extremely wide range of fish. They range in size, aggression, diet, and region of origin.
First off, anyone who gives you advice for your aquarium that is BLACK or WHITE is wrong. Don't listen to them. I can't tell you how many times I've heard of hobbyists who have kept predatory fish with small guppies and have never had a problem. The only rule to really go by is that someone, somewhere, has an aquarium that will prove everything you declare WRONG.
Keeping an aquarium is an ongoing experiment. Personally, I don't believe that the combination of fish that you have will be the best option for you. From my experience, I would say the Ghost Knife has a taste for fish. You're just lucky that your Platy
swords and rainbows are larger than his mouth. Knife fish also have a solid spine, so they do need a wide tank to move around in, so a 75 is kind of a fringe area. You may need to upgrade in a few years or sell him back to a LFS. And NO they will not only grow to the size of the tank. Fish grow just like us. There is no magic spell that makes fish stay small... Same with your pleco. A Pleco is a pretty cool fish, but contrary to many people's belief they do not aid in cleaning your water. They just make it worse, just like any other fish. They cannot remove nutrients from the water, that is done by natural bacteria growing on its own, and through your water changes. While common
plecos eat algae when they are small and young, they quickly grow to two feet and terrorize your tanks decorations and plants... they also mostly stop eating algae. One would be okay in your 75 gallon though. Just beware of the live plants issue. To take care of algae, choose the smaller bristlenose plecos. They stay about 4 or 6 inches and usually don't bother plants.
may be the biggest problem you run in to. The Oscar
is a South American Cichlid that can sometimes pack an attitude. This is not just because it is a cichlid. Yes, many cichlids usually are aggressive and territorial. But saying "cichlid" is kind of like saying "car." While you have an Oscar
, a cichlid, it is very different from the angelfish, also a cichlid, in terms of aggression and territoriality. Discus
are cichlids as well! Many people often assume cichlids = overwhelming aggression, which is not always true.
Now, being a larger fish that will probably reach a size of a foot or so, you have to be willing to experiment. I would recommend fish of about the same size... A fish that will be able to "hold its own" in a tough pecking order. It is possible to keep several large and more aggressive cichlids in the 75 gallon, just understand that not everything is going to work out. There will most likely be a "boss" of the tank, that may or may not be your Oscar
. Some other fish that may work out would be a Jack Dempsey
, Texas Cichlid
, Red Devil, or Green Terror. I would just recommend sticking with LARGER South or Central American Cichlids that commonly are more aggressive.
The platies, swords, rainbows, and redtail shark should be able to get along together in a community aquarium. The redtail shark sometimes can be a little nasty, but you could definitely get a peaceful one! I would add more types of rainbows, and several large schools of fish. If I was working with your 75 gallon, I would definitely add a TON of live plants. Because plants are awesome and beautiful and only benefit your aquarium. I would then add about 18 Cardinal tetras, 12 Harlequin Rasboras, and probably quadruple the same number of platies and swords you have in there already... but that's just me! This is YOUR fish tank. Just make sure to take it easy. Try to cycle your aquariums for the next month or two ONLY using the fish you currently own. Keep up on your water changes, and don't be afraid to do an extra one in the middle of the week for now. If you see your fish stressed, and aquarium, non-iodized salt to your water as a general stress reliever and immune system booster.
But please. Remember one thing... Have FUN!
...and do water changes. It's honestly the solution to every problem.