Also, a question for Colin, I'd prefer to have some kind of soil as the bottom layer in my tank, and the sand on top.
If you try to have different layers or types of substrate, you will make a mess and the fine sand will simply fall through the gaps. Only use one type of substrate, sand or gravel but do not bother putting anything under sand because it will simply mix.
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Grow plants in pots if you want them to have lots of nutrients. (see post #8).
Yes you can buy plants that are already in pots instead of buying them from the other shop.
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The 2 Arowanas will fight and the big one will probably kill and maybe even eat the smaller one.
If you do not have another big tank, then do not take the second Arowana from your friend.
Arowanas will eat anything that fits in their mouth. Bigger fish can be fed on white bait, prawns, squid, etc. Buy a packet of frozen whitebait, blue sardines & prawn from a fishing shop. Keep them in the freezer. For smaller fish you take out 1 or 2 white bait, blue sardines or prawn/ shrimp, defrost them, cut them into pieces that are small enough for the fish to eat, and offer 1 piece at a time.
When you feed prawn, defrost a prawn, break the head off and remove the shell, and pull the intestine (thin black thing) out of the prawn. Throw those bits in the bin. Then cut the remaining piece of prawn into bite size pieces and offer 1 bit at a time.
Your stingrays will also eat these foods and can be offered smaller bits.
Remove any uneaten food within 5 minutes of feeding.
For big Arowana you do the same thing but you don't have to cut the whitebait or shrimp into small pieces. A 12 inch Arowana is able to swallow a 3 inch long fish like a whitebait or blue sardine.
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Some good plants to try include: Ambulia, Hygrophilla polysperma & ruba, Ludwigia, Elodia, Hydrilla (looks similar to Elodia), narrow Vallis, Amazon swordplants. You can also try Banana lily leaves or Water Sprite (Ceratopteris thalictroides) for the surface.