as with salt water there are a thousand different directions you could take but a simple hang on back filter would do the trick for most smaller tanks with a lite bioload, you wont need any fancy lighting or any other crazy equipment unless you get into more specialized stuff like planted tanks or more advanced fish. most basic filters and heaters will get the job done for easy community fish like guppies platys and various tetras. just keep up with your maintenance.
if you have a saltwater tank that you are converting you can use a lot of the stuff as is. like if you have a reef ready tank with a sump with filter socks, a heater, a skimmer, carbon/gfo reactor and a return pump you can sell the skimmer and use everything else, it would be overkill but it would work.
I've had a 20 gallon freshwater tank for about 3 years. It has gone through many changes, starting with fake plants and multi-color gravel, to its current state as a planted tank. Like @picoreefguy said, if you have a sump already, use that, otherwise I recommend a canister filter. Hang on the back filters work, but they don't allow for much filtration media. I recommend a course, medium, and a fine sponge and/or filter floss/pad for mechanical filtration, and seachem matrix or biohome media for biological filtration. I don't use any chemical filtration media. Add lots of plants and get fish, shrimp, and snails that will help keep it clean and algae free. My worker creatures are ottos, nerite snails, ammano shrimp, and cory cats. My plants are all low light plants that are thriving under a cheap led light. If you want to go with higher light plants, then you will need to look into injecting co2. I just dose a little liquid co2 once a week. Many people dose liquid fertilizers as well, like Aquarium Co-op's Easy Green. Like a reef tank, it is all about finding the right balance. And yes, for freshwater, tap water is fine, just be sure to add something to neutralize the chlorine like seachem prime.
I have a a 50 gallon cube with about 20 gallon sump. I am thinking of keeping a little of the reef rock and adding some kind of wood with plants. For the fish probably a couple of glow fish maybe a cat fish and was hoping to get a small puffer and of course snells, shrimp, maybe one of those frogs if i can sneak it by the wife.
I only ever used prime except for my constant water changer on my 480g I ran through two carbon blocks. I never noticed any issues with any of the water. I did, however, feel like my most recent 210g FW tank had Ph issues. I may have had issues at my old house with the 2000g of water I had in my basement but the 210g in my living room didn't keep the fish happy. I am pretty sure it was a ph issue. Something that I think may have been the cause of death of many rays a few friends and I tried to keep. When I really started looking in to it it seemed like our very low ph, which is good for rays and most of the South American fish we were keeping, was also very low in alkalinity. I think normally that's an issue with ph drops, which it could have been, but I think it's more likely that it was spikes that they didn't like.
Plants don't need a lot of flow. Also, if you have too much surface agitation from a wave maker or air stone, you will also lose CO2 which the plants need. Most people aim for around 10 times turn over and you could even go up to 20 times. I would handle the flow with the return pump.
I always used tap with my FW tanks with Prime. I also used Excel for the plants which seemed to help. I used T5s over my planted tanks and they responded well. No wave maker needed, if you use a HOB filter or canister you’ll get all the flow needed.
Ya tap water will be fine for most freshwater unless you have excessive amounts of metals in the water. Just mix in prime in a bucket instead of pouring it straight into the tanks. Canister filters are fine too. If you have a sump just toss some media in there