So what’s the safest, most stable and most effective way to decrease your PH for an Arowana?
There are 3 main "safe" and effective methods that can reduce the pH in an aquarium the real question is do you really want to spend money to fill that bottomless pit?
1) As
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phreeflow
stated you can use RO water mixed with your tap water to reduce the overall pH. The issue here is that your tap water comes out a bit hard which will hasten the degradation of the your RO unit's filter = more new filters = more $ spent. Also RO/DI units should not be used ideally.
2) A cheaper, although questionable alternative depending on where you're at, is storing rain water and using or mixing that with your tap water. Remember to strain your rainwater for any inscet larva or other large solids.
3) Finally you can use many acidic products ranging from, but not limited to, vitamin C to hydrocloric acid in a separate tank and mix that with you tap water to reach your ideal pH level before doing water changes.
Always add the acid to the water not water to acid.
All 3 options must be performed every waterchange but 1 & 3 will cost a hefty sum of money overall while option 2 is a dice roll. However, all 3 options will require additional labor and effort on your part. If you're still willing to go through with all of this effort to chase pH then option 1 is the easiest, on demand option. Option 3 may be the cheapest option depending on how large your tank is and what acid you use. Option 2 is the cheapest option and potentially the best depending on your locality but you will also probably have a lot more mosquitoes around...
Finally, I would again suggest not chasing around your pH and just keep using your tap water. The only time I suggest increasing/decreasing pH is when values are above/below 6/8.5 respectively. This is because nitrate/ammonia respectively start becoming toxic at those levels.