had loads of plants but they all got eaten
A common misconception. The plants died and the fish were picking at the biofilm. Big fish, and I mean big, can do a lot of damage and eat plants, a large plecostomus, say 18 inches, will make a meal of most plants. Tinfoil barbs are notorious and definitely eat plants, small tetras, barbs, gouramis, livebearers and snails no. Even the more modest sized fish that will eat plants - and there are some, for example the Chinese algae eater - can't keep up with the growth rate of healthy plants, a tank is like a garden, you get snail damage but hedges, lawns, trees and many herbaceous plants grow too fast to be seriously affected, though hippos and elephants will wreck gardens and eat a lot of vegetation in some parts of the world, just correcting myself there! And I now recall that a friend I was staying with once had a cow in a garden, it did I admit, do quite a bit of damage.
You need good light for a fixed period every day, approximately 10 hours, good substrate, careful use of nutrients and I suggest a really good book - Sunken Gardens by Karen Randall is I think the best currently in the easily accessible category.
Since you are Hampshire. Easy plants for hard water: hornwort, egeria and a little more demanding vallis and hygrophila polysperma, if you really are struggling to grow anything, duckweed.
Given your location you may need to add small amounts of sequestered iron and I mean small amounts, when doing a water change, if you stain the water brown you've added too much.
Rocks in streams get green algae on them, its natural, realistically not much you can do about it. Show tanks with clean rocks are meticulously cleaned, if you follow some of the Youtube content you find video material of folks using wire brushes on rocks to keep them pristine. Such activities fall outside my Zen zone! Even the Buddha in my garden, surrounded by large rounded rocks has a view of lichen, algae and moss. Such is the wonder of life.
Good luck.
