I'd make a 1:1:1 mix of bark, pumice, and perlite.
Instead of pumice, you could also use Turface, or Napa diatomaceous earth, part #8822. These ingredients are similar in that they have a solid structure and great internal porosity (to hold moisture and nutrients).

I use quartz gravel and perlite together - the gravel gives some weight to the mix, while providing excellent stability and air-circulation and while holding very little moisture. I've also read that rough, sharp grit in your mix will promote fine root-growth.

The bark that I use is called Micro Bark: it is pine/fir bark. I screen my particles so that they are between 1/8 and 3/8 of an inch. Ideally, however, all of my ingredients would be just about 1/8 of an inch. This is the critical particle-size at which a container mix will retain moisture (under 1/8 holds water - over 1/8 gives up water). This is a general rule, assuming equally-sized particles.

There are many ways to modify this basic blend to get the mix right for your growing conditions.

Since Pachira aquatica (Malabar Chestnut) comes from wetland areas in South America, I have always assumed that it likes to be flooded...and then allowed to dry. Outside in partial shade, my Pachira is drying out every 2 - 3 days. During the winter in the house, I cut watering back to once a week (sometimes longer).

Oh! Almost forgot a final piece of advice!
Avoid the braided Pachira...in my opinion, they're nothing but trouble (often, one of the trunks is totally rotten from the start). A single trunk, with healthy foliage, will serve you better.

Josh