The smallest arowanas are the Scleropages species (Asians and Australians) which rarely grow larger than 20 to 24 inches in captivity, but if you live in the US you will be limited to just Australian arowanas as the Asian arowana Scleropages formosus in all its colour forms is illegal to own. This pretty much only leaves you with Scleropages jardini which is an extreemly aggressive species which usually requires being kept alone.
Black arowanas from the Osteoglossum genus (South American arowanas) generally dont grow as large as silver arowanas in captivity, again usually not exceeding 24 inches but be prepared for an animal which could get much larger. Silver arowana should be avoided unless you can provide a huge inclosure from the start, well kept specimins easily reach 30 inches and can grow at a rate of 2 inches a month for the first year.
Some people believe that adding black water tonic to the water brings out the colours more intensely but it certainly not a nessesity to keeping arowanas, i keep my 3 arowanas in plain London tapwater which has a pH of 7.8 and a KH of around 15.
Large water changes go hand in hand with any large predatory fish, large ammounts of waste require large changes to dilute the toxins down to safe levels, 40% of the tanks volume weekly is recomended.
A pH anywhere between 6.5 and 7.8 is fine, just avoid the extreems at either end of the scale, below 6 a chemical change happens which converts ammonia to ammonium which is less toxic but can play havock with the nitrifying bacteria in the tank and very few tropical fish appreciate a pH of 8 or above. The main thing is to keep the pH stable and dont let it fluctuate up and down, unstable pH is probably the biggest killer of fish in mature aquariums.
