, I too took my first steps into the planted aquarium world with a tank about that size (5.5g). I learned a good bit from it, though it's been neglected for quite a while due to stepping it up to a 29g now that I know a bit more about what I'm doing. Here's some of what I learned from that first tank:
If low maintenance and simplicity is what you want, then low tech is the way to go. This is what I did, using 14W T8 over the 5.5. You'll be limited on plant selection, but there's all kinds of nice plants that will thrive. Anubias nana petite will and stays an appropriate size for the tank and grow just fine, if slowly, marsilea can be used to carpet if you like that look, etc. Pellia has done really well for me personally. Injecting CO2 isn't necessary, nor dosing ferts. I set up my tank with Eco-Complete, added some fish and shrimp and that's it.
For filtration I have a rapids mini canister filter set up. I think it's rated at 80 gph, and the flow is plenty sufficient I found. I have ceramic bio media and filter floss in it, and haven't worried about it, cleaned it, or had a problem with it since getting it running. Some people seem to have had bad experiences with this filter, but nothing has gone wrong for me. You might want to keep that in mind though. Also, the more heavily planted the tank, generally, the more flow you'll need. Ten times the tank volume in gph is usually fine. You can always add a small powerhead down the road though.
Another good online supplier to maybe check out or consider would be bigalsonline.com
. I know many people here use and are happy with them.
Anyway, I typed a lot and hope I helped out some. I've never kept a reef, but I'm really liking my planted tanks, and this hobby. Let me know if there's anything I forgot, and good luck. Ah, and welcome to TPT!
Edit- Caught another question. I use treated tap (well) water and have been doing just fine.