dam , sorry to here you lost him mate , after reading later posts than mine ,i think that maybe the reason you lost him was an ammonia spike from adding the new filter ,a arow at that size it very delicate , you should have much more luck with a larger specimen ,BUT before you buy him make shure you test your water for ammonia and nitrite and make shure they are ZERO if unsure pop a sample into the local shop and get them to test it. also have some sort of plan for when he gets bigger ,as you probably know a 125gal is not going to last for life, so make sure you can either supply larger tank when he grows or have someone who is willing to take him off you , rehousing large arowana is not that easy , also ask to see new one eating in shop ,make sure it looks perfect, and not thin no missing barbells or scales ,silvers are easy to come by and you can afford to be picky ,this will also give you the best chance of success with it , find out what the shop has been feeding it ,and do try to get it on arowana pellets then just feed feeders as treats very occasionally , personally i never feed feeders but that choice is down to each individual , also if you haven't bought your 125 yet ,have a think about it and maybe get a custom tank built by local tank builder (sometimes can even work out cheaper than a shop tank often not much more expensive) don't worry about height but get the widest tank you can afford / fit in your room , i.e a 3ft wide 3ft long x 18" high would be much better than a 6ft long by 12 inches wide by 3ft high ,obviously the more floor space the better though , good luck with what ever you decide on