Hi StickyEmu,
Your tank is 90 litres = 90,000ml
You want to raise ammonia level to 5ppm (parts per million), so
90,000ml / 1,000,000 x 5 = 0.45ml of pure ammonia is required;
Most ammonia solutions are 9.5% ammonia, so i'll go on that basis, but you should check the bottle before relying on my calculation.
0.45ml / 9.5 x 100 =
4.7ml of 9.5% ammonia solution required to raise 90 litres of water to 5ppm.
If you're mathematically minded, this can be expressed by the equation
A = (0.5 V) / P
where;
A = Ammonia solution required in millilitres
V = Volume of tank in litres
P = Percentage of ammonia in solution
If your ammonia isn't a 9.5% solution, let me know and i can re-calculate.
Hope this helps.
BTT