depend on what type of gar you are keeping , for a spotted or florida gar , 300 gallon would be adequate , for an alligator gar , better get an outdoor pond , for temperature , gars are temperate so an unheated tank should be fine as long you dont keep any tropical with them as tankmates . for feeding , you can first try floating hikari carnivore pellets , froozen bloodworms if gar is small , deforst shrimp etc , look at what the store owner are feeding them , some are pellet train from the store already . If they dont accept dead food , try to feed them livebearers ,like guppys or gambusias , size of feeders will depend on the gar size , I use to feed mine fish a little longer than their snouts .
as for ornaments , a bare tank will do , but if you want to add any ornaments , keep the top surface area clear , the gar will often cruise at the top , you can add some driftwoods at the bottom of the tank ,some plants etc , gars are not very maneovrable , so give them enough space to turn around comfortably .
They are very peaceful with any fish they cannot swallow , just try to avois keeping them with aggressive territorial fishes like some cichlids .Plecos are to be avoided too as some tend to suck the gar's side which can cause infection .
Also remember not to overfill the tank , they need to breath air regularly , you will see them rise take a breath from time to time . keep the tank covered too as they can jump when spooked .
I hope I have covered anything you need to know , feel free to ask any question .
