Apologies if my responses are all a bit late.
What’s the average investment cost for an aquarium, not just the tank but also the required equipment and everything?
Deciding between an Aquarium or Terrarium in the future since my plans for a pet pig are not feasible for now.
I've gotten several aquariums just off of the side of the road(a 100g(had leaks, so I disassembled it and plan to remake it with a wooden frame), 20g, 10g and a 3g). You can do both at the same time if you have a big enough container(it'll be called a "paludarium" then), but terrariums are a lot easier if you want something easy to manage. Just having a plastic tote with some holes either drilled through or melted through with a soldering iron(do this outside and preferably with a fan to blow the smoke away). It's advisable to cover up the holes with something like breathable medical tape or some kind of cloth or screen to prevent flies from getting in or out.
You can get isopods or other critters outside to start out, that way you don't need to invest too much either. Dirt is dirt cheap and you can get moss or small plants to grow in there too if you're having it exposed to light(avoid sunlight for anything plastic, shop lights are cheap and don't produce UV light that will break down the plastic and make it brittle). There are tons of YouTubers out there who make terrarium and aquarium builds and I highly recommend
SerpaDesign for some inspiration. He also has a video going over moss collection and other tidbits for terrariums:
Do goldfish need friends?
I don't remember them ever needing friends. Usually it's smaller fish who school that are really anxious without others of their kind. I've seen goldfish just doing fine on their own.
I have no idea if his methods are at all usable. I just think his whole ‘everyone else on the internet is WRONG’ schtick is funny.
I think I've seen this guy, or maybe it was another old guy, before that had a nice video talking about deep substrate(the sand and dirt on the bottom). Deep substrate is actually useful for helping to lower waste products as you start getting anaerobic microbes of all sorts of varieties that help out. If the substrate is also getting organic bits into it(or there's organic bits to decompose down there) then you can get
denitrifying bacteria in more anoxic dirt that'll consume nitrates like we would with oxygen.
You can get away with not needing to do water changes though if you've got above water plants to soak it up. You just have to trim the plants every now and then.
I’d want to give my fish their best life so any recommendations for getting started? I’d want to start small.
You probably want fish that can go for a good amount of time without food. Now a lot of fish can do this, but generally not fish who live in warmer temperatures(hotter temperatures means their metabolism runs faster, temperature dictates the rate of their metabolism generally). Good options would be to get fish that can live in colder temperatures and to have a water chiller that you can use when you're gonna be gone for a while and a timer for the light(s). Tropical fish are likely not gonna be an option for anything like that.
Most fish in general at cooler temperatures don't need to eat a lot though and a lot of fish may be able to go for a good long time without food. Another good option is to breed your own live feeders for your fish and when you have to go you just add a bunch into your tank. So long as those feeders can survive in the tank on their own then they'll persist and be a food source for a while whilst you're gone. The feeders that'll live the longest in a tank will largely be small crustaceans like scuds(amphipods), clam shrimp and daphnia. Scuds are easy to culture since you can feed them tiny bits of food scraps and brown paper napkins and bubblers don't tend to be an issue for them like they can be for the other two mentioned.
As for tips for aquarium stuff, I highly recommend having something like a pothos cutting that you can have floating in the water of any freshwater tanks. Either have it hanging from above so that its roots get into the water(or the cut end can) or have it set on a float. They'll readily pull waste products out of the water and the only issue I've had with mine is that it grows constantly. It has 4 vines that are over 15 feet long, the longest one is probably closer to 22 feet, and I plan to have the main plant planted outside soon with a newer cutting to take over.
You can use a variety of plants to do this, but I recommend pothos mainly because I can vouch that things like bladder snails won't eat them. I've floated other cuttings in that aquarium but the strain of very ravenous and mutated bladder snails that I have chew up every plant that isn't poisonous to them. If you don't have to worry about anything eating your plant cuttings though then aquariums make for great nurseries for any plant that can be made to grow roots in water.
