Get a plain normal Betta with the long tail but don't buy a fancy variety like dragonscale, koi, or anything else that costs way more than it should. The more expensive and unusual the Betta, generally the weaker they are when it comes to their health and lifespan.
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They should be fed a variety of foods ranging from frozen (but defrosted) brineshrimp, mysis shrimp, bloodworms, prawn, marine mix (prawn, fish & squid blended up).
They can had live brineshrimp, daphnia, mozzie larvae, aphids, small flies that are free of chemicals.
You can give them a dry Betta food as the main diet but add frozen and or live food every day in addition to the dry food. Dry food in the morning, frozen or live food in the later afternoon.
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The best floating plant for Betta splendens is Water Sprite (Ceratopteris thalictroides/ cornuta). It grows well and has lots of branches for the fish to rest on. The plant also grows well underwater and can be planted in the gravel.
You don't normally need rock or wood caves but there are floating betta logs, which are plastic logs that have a hole in them and they have a suction cup that holds them near the surface. Some bettas like these.
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If you already have an established aquarium, take some of the filter media/ material from that filter and put it in the new filter for the Betta tank. It should give you an instant cycle so you don't have to wait 4-6 weeks while the new tank cycles.