I've used the API PH 7.0 brand for years with betta, regular fish, and plants. I add it as needed when I do my water changes. Most plants will do well in the PH ranges from 6.0 to 7.8 Some will do well at 8.0. I add C02 API supplement daily to my water for my plants and even the ones that don't need it do very well. I also use SeaChem flourish once a week as a Fert. I don't use the flourish excel because of the added iron. my water is hard enough. Even with a tap reading of 8.0 The API PH 7.0 will bring the PH balance to 7.0. However with your readings with the rain water of 6.8 - 7.2 There really is no reason to raise the level. With betta a PH reading of this type will not hurt them. As for not even being able to grow algae, my question would be on your lighting. If your tank came as a kit and the hood is LED white light You would have a low light situation and would do well with plants like Anubis, Tropical water fern, Java fern, most bunch plants. If you want a better verity of plants increase your lighting to something like Finnex Stingray or the Finnex planted plus. [url=http://WWW.finnex.net] The site to buy this lighting from is www.aquavivrant.com
. Some of the best lighting at a really good price.