Since you have hijacked this thread I will try and answer you. The point of being a practical solution to a problem very much matters!
As far as biological denitrifiers they won't work in a pool because:
1. they require a very slow flow or stagnent water (the process is done by anerobic bacteria)
2. chlorine would kill the denitrifying bacteria
3. it is an extremely slow process that would literally take weeks to months before the nitrates were significantly reduced
as far as reverse osmosis, it won't work in a pool because RO units have very slow flow rates and cannot process large volumes of water and there is much water that does not pass through that would be discarded as wastewater so you would lose a large percentage of the water even if you had a unit big enough to process your pool and would be refilling with other water anyway
It is also a slow process
as far as ion exchange,
the resin would get depleated and need regenerating rather quickly. since this type of resin is regenterated with a brine solution you would have that to deal with (which is also the reason it would not work with a salt pool, the salt water would just release any trapped nitrates from the resin back into the pool water)
The flow rate through the resin would need to be much slower than the flow rates in your average pool and the resin tank would need to be very large to accomodate the amount of water. The cost would be prohibitive.
this once again leaves dilution as the only PRACTICAL method to reduce nitrates in a pool.