Sands good for cories, i wouldnt buy it from a pet shop though, can get a 15kg bag of play sand (that looks nicer than most pet shop sands imo) for like 3 quid. Only prob with sand and plants is untill they root they can be hard to anchor sometimes, ive got some smooth gravel mixed in with mine.
Sands fine, what did the shop say bit confused? Best way ive found to put the sand in is (wash it obv) then put in a bag you get your fish in, grip it so its tight take to the bottom of tank and let it out slowly, will stop so much of it getting into the water. Sand also makes cleaning a bit easier as the poo and uneaten food sits on top just easy so siphon.
And with wood just get whatever u fancy (if its unused soak it in hot water for a few days, reheating the water twice a day or something) to get some of the tannins out, some will still get in water will clear eventually with w/cs tho, could also look into what trees you can use (like going down the forest and finding some) and properly preparing them for the tank (never done this myself but have seen it done, with berch or something) which looked good cos they where branches rather than lumps of wood.
hope this helped
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