I recently had a problem with my GH and KH being very low, and I definitely wouldn’t reccomend adding any animals to your tank until you’ve raised those a bit. The way that I raised mine was by using cuttlebone and Salty Shrimp GH+. I would recommend raising your parameters slowly, so that your plants have plenty of time to acclimate to the change. Maybe a 5% water change with the new water every couple of days. Get an API liquid test kit to test your GH and KH, and mix up your new water until you’ve got it where you want it based on what exactly you’re planning to add to your tank. As long as you don’t change your tanks parameters too rapidly, your plants should be able to acclimate well, and with a more bioactive aquarium they might even thrive better once you’ve got some critters living in there! More fertilizer for them! Just remove a little water from your tank and add your new water little by little every couple of days to give your plants time to acclimate and you should be good. As for what water to use, I suppose that depends on the parameters of your tap. If your plants thrive with what you use now I wouldn’t change it, but if the expense is a problem then I would treat the new water the same way, just mix it in little by little, so your plants can acclimate, and observe the effect. If they start having problems, then I would just stop and resume what you were using before. If you do decide to go with tiny fish like tetras, might I recommend also adding some Neocaridina dwarf shrimp as well? They will help control any algae development, and help manage any waste from leftover food that you give your fish. Neos especially are pretty hardy, so they’re not too difficult to take care of either. I also like to keep a few snails in my planted tanks as well, but that would depend on what plants you’re using, as certain snail species have a tendency to eat some aquarium plants when there isn’t enough food for them. Good luck with your new tank!