Hi shrimpfoot,
When your water comes out from your RO unit, the ph will be low due to the process that its gone through in all of the different stages and this water is still full of co2, and the reason for the co2 will make your ph low, like 6.
if you let your water stand for a day or two, or aerate the water the co2 wil gas off and you should have a ph of around 7 - neutral water.
it wont be the case that your ro water has a ph of 6 and aquasoil will drop the ph even further due to your ro having a ph of 6.....if that makes sense
a ph of 5.3 is quite low yes, but definitely do-able with Caridina shrimp.
i run all my tanks with aquasoil. 2 9L bags in around 130L of tank water and my ph stabilizes at around 5.5 ph.
how long have you had the BB in this tank?