Bamboo Treatment Tank Maintenance

I'm trying to maintain the concentration of a Borax/Boric acid in our bamboo treatment tank. We add additional salts as we top up with water due to the level drop, which must be part due to absorption and part due to incomplete drainage of the poles back into the tank, but after some years and many batches I'm concerned that the solution may not be at optimum strength.

I have not found using a hydrometer or measuring ppm to be effective, I guess due to sugars etc leaching from the bamboo. Do you have any advice? A rule of thumb calculation? A suitable measurement device? Or should we just continue to go by drop in fluid level?

Additionally our tank is black and smelly although we circulate the solution through a bag filter and occasionally add iodine. Do you have methods of keeping the solution cleaner?

We are in the Philippines, using primarily "Butong" Dendrocalamus asper for construction. We have a 1M X 1M X 11M tank in which we soak 20 to 25 10mtr poles, 2 weeks per batch. The tank is covered. We have a starting solution of Borax: Boric acid 75KG:50Kg in 7.8cu.m water.
Any advice would be much appreciated.