Sand on top of some form of aqua base will grow plants fine. Unless your adding pressurised co2 the lights will be fine just keep to slow growing plants. Also I have found that some carpeting plants find it harder to root through really fine sand which can become compacted. Go for a slightly larger grain size. What fish do you intend to keep? If the rocks are releasing minerals then it could increase water hardness. Test the water in the bucket before and after adding to see what the impact is?
Hello Generally speaking unless it's salt-laden or calcite (eg limestone, which raises the acidity of the water and can affect the chem balance) it's okay. As for plants, CO2 and fertiliser are vital, otherwise they just die and add to the bioload in the tank. Plastic plants can look weirdly efferctive if they're arranged well... Have attached a pic of mine - let me know if it works
maybe i'm missing something but it looks like normal slate?
There's a sequence of rock formation that begins with clay/mud ... which gets compressed to form mudstone of varying solidities ... which then becomes mudstone with feeble layers .. and finally, after a lot more compression, becomes slate. My guess is that it belongs somewhere within that sequence. As it's essentially clay, it would be unusual for it to pose a health problem for fish (unless it were obviously contaminated with (say) heavy metals) but the less it's been compressed, the more likely it is to 'muddy' ones water! The shimmering specks are likely to be mica or pyrites ; neither would be problematic (even though the latter contains sulphur, it's quite stable under aquarium-type conditions, I think - but do double-check that).
Whether the vinegar is malt or white shouldn't matter. re : the GH kit. Yes - I remember not so long ago I went to #1 LFS and begged a quick GH test (I'm always begging them for such stuff!) because I'd run out and couldn't justify buying a new refill. They lent me a bottle of reagent and I bore it triumphantly home. It didn't work - after about forty drops there was still no colour change. So I went back and they dug around in the chaos that is their bin full of test kits and came up with another. That didn't work either : same result. They cracked open a new one and that worked perfectly. It was impossible to tell how old the non-working kits were but it was obvious that these GH kits become more unreliable as time goes by until, at last, they conk out altogether.