I've said this here before many times but I'll say it again, put a layer of soil under gravel and your plants will thrive. Stock up with enough fish to provide plenty of carbon dioxide, they are the best and most interesting source. I've had tanks like this for 3 years now, minimum maintenance, plants grow like crazy, fish healthy and happy. It costs very little in money and time. Let the fish poo sink into the gravel, that's plenty of fertiliser. Of course, if you obsessively vacuum the gravel and change 25% of the water every week, you are going to be doing a lot of work and will then have to replace the nutrients you are removing. Apart from topping up, I haven't changed water for three months or more, I have no problems with fish health whatsoever, they are all lively, brightly coloured, eating like pigs and thriving. In those three years, the only additive I have used is water conditioner. Filter maintenance is the only regular task, apart from feeding. I have to remove excess plant growth rather than try to encourage the plants to grow.
The tanks I set up with sand are not as good, the fish poo simply lies on top, doesn't sink in and has to be removed. I'm changing those around this winter and will replace sand with gravel. The gravel will again be lime-free horticultural grit from the garden centre at a fraction of the cost of "aquarium" gravel, which is basically the same stuff in smaller bags at an extortionate price. In the sand tanks, I presently have large pieces of bogwood on which Java Fern, Java Moss, hornwort, Anubias and Hygrophila polysperma all do very well tied on, not rooted in the substrate. There is plenty of oxygen for their roots in the water which is lacking in sand, another reason to prefer gravel. I've not seen any difference in Corydoras health in sand over gravel, they don't seem to mind either way.
Algae are much more likely to be due to excess nutrients rather than light. Cutting back on light is more likely to slow your plant growth rather than reduce algae. Fast growing plants use the nutrients and outcompete the algae and so you want your plants growing strongly.
Any plant will grow in hardish water, given suitable substrate, but hard water plants will not thrive in soft water. I think the advertising authorities should ban "liquid carbon dioxide" as a description, it really isn't true. Most plants will use bicarbonate ions, naturally present in hard water, and any cheap source of bicarbonate would do as well as the commercially promoted expensive stuff.
To quote Diana Walstad again :
"…..... there is little to be gained from restricting the nutrient levels in the aquarium by underfeeding fish, changing water frequently, and cleaning gravel, but then adding it all back as plant fertilizers.(Sounds like a lot of work to me.)"
Apologies to those who have heard all of this from me before, but I know there are a lot of new people on the forum.