I have had my 'Nickel-wall' batteries for 6 months now, and I need some definitive advice on battery maintenance please!

They are four 200Ah 12v each, arranged 2S-2P to give 24V 400 Ah. (nominally 10kWh). All are about 4 years old, so getting tired.
As they were originally big generator (2MW) starter-batteries. I do not deep cycle them, but use a maximum of 10% of the available capacity, i.e. less than 1 kWh per charge/discharge cycle.
The electrolyte level is the same in each cell, and I have recently topped them up using deionised water from halfrauds. (£3-ish per 5l pot).
They take 200 cc per cell to raise the electrolyte level by 10 mm. The plates are still covered by 20 mm, but the level is still below the minimum marker on the cases.
Now the questions:
How much juice do I put in at a time?
Do I add after or before charging?
Do I run the battery low before top-up?
Please steer me in the right direction, I know they are old, but I'd like to eke 'em out for as long as possible, due to 'funding issues'.
Do I need to buy a cheap hydrometer?
Cheers all!
