Thanks for providing the additional details. I have a 2 year old son, many many hugs to you.
So with regards to the philosophy of fishkeeping, there are a few different schools of thought. Some folks say you should do 50-75% water changes each week, and others are all the way at the other end of the spectrum bragging about how they never do water changes. I personally fall somewhere in the middle, and I'm a firm believer that each tank is different and needs a bit of a "custom" solution. The basics of fishkeeping chemistry are that fish generate ammonia, the ammonia gets turned into nitr_I_te by beneficial bacteria, which then gets turned into nitr_A_te by other beneficial bacteria. The main reason test strips exist is to test for these three things. If your ammonia or nitrite ever read above 0, it can be trouble for your fish. The nitrate can read above 0 without killing your fish right away, but it can also become toxic to fish in high concentrations and/or over time. The nitrate part is why water changes are done (simplified version). Over time, nitrates build up in the tank, and you do a water change to get them back down. There's other stuff that happens too that causes needs for water changes, but I'm trying to not information overload you.
The ways to improve your "water health" -> fish health is by doing frequent water changes and having lots of plants. My personal goal due to also needing to live a life outside of fish responsibilities is to do a water change about every two weeks, and to try to keep the water "healthy" in the interim by having many plants and not keeping a lot of fish (fewer fish -> less nitrate build up).
All that being said, you should be able to have more than two fish in your tank! Do you have any plants? Some things that may bring out aggression in fish include unhealthy water, lack of hiding places/shelter, or stress from other tankmates.
It sounds like what you have been doing so far is ok, so we don't have to make any huge changes right away, but I agree that we should support you in your goal of figuring out what's going on so that you can add more fish and keep the existing inhabitants healthy.
Here are a few thoughts that come to my mind:
1) fish food: The tetra flakes are fine, but some people advise varying up the diet a little bit such that maybe 4 days a week you feed the flake food, then two days a week you feed pellets, and one day a week you feed a "treat" like bloodworms (these are freeze dried) or similar. The thinking behind this is that fish that are fed a varied diet will thrive, and as such will likely show more species-native behaviors as opposed to stress behaviors.
2) water parameters: I went on at length about the cornerstone nitrogen cycle of aquariums above, but there's one additional thing to consider that is relevant when planning on adding new fish. This is the idea of water hardness. I see that you are in the UK, and from my understanding it is predominantly good to assume your water is relatively hard. However, I am tagging
@essjay here so she can help you figure out what your actual water hardness is at your location (to get a head start for you, look up your water supplier and see if their website has a water quality report that lists hardness or "alkalinity"). When you get to the point of figuring out what fish to add, it is good to check a website called seriouslyfish.com (Search for your fish of interest on there) to determine if the hardness that the fish is adapted to matches your water hardness. In the long term, this decreases fish diseases and allows your fish to live longer.
3) plants: I put this item last, but this is probably what I would start with first. I'm not sure how much of a "project" you want this to turn into with everything going on (or, it might be a welcome escape, completely your choice), but some easy plants to add to your setup are called java moss, java fern, and hornwort. there are other easy to keep species as well (I think anubias falls into that category too), but there are people who are plant experts here who can help with that. The reason I am advocating for plants is that if you don't have a chance to do a water change one week for whatever reason, or similar, the plants help your water quality tremendously and can help prevent any instant deteriorations in water quality if life gets in the way. It's better to try to stay on a regular schedule, but we all know that life.
I think I have written quite a bit at this point, so hopefully this is helpful and not overwhelming for you. I think the key things are to just take things slow, and we will figure out how to resolve the angel problem and get him/her some tank mates that he will play along well with. OH, one other thing to consider: depending on how old your angel is, you will have to at some point consider a bigger tank for him. It is possible that the aggression is coming out because he feels cramped. Seriouslyfish recommends that the minimum tank length for an angelfish is 100cm so that they have enough room to swim, so it's an idea that I want to plant into your mind because for the health of the fish it is probably something you will need to think about sooner or later.
Here is the angelfish profile if you are interested in reading it:
https://www.seriouslyfish.com/species/pterophyllum-scalare/
I know I just wrote a ton, I hope I didn't overwhelm you - remember that we here on the forum are really into fish, and you don't have to be as excited about everything related to fish as we are in order to be successful in the hobby
I just don't know anyone who has a fish tank or to come and pick him up x
