JBL The 7 balls (Aquarium Plant Fertiliser)
I can't get all of the balls out of the tank. Is that harmful to my fish?
The balls don't contain any heavy metals, so that they are not toxic to fish. Therefore, there is no need for concern if a pellet or two stays in the tank.
If I am using JBL 7 balls, does it make sense to also fertilise with JBL Ferropol ?
Yes, that will be helpful and may even be advisable, depending on the plants in the aquarium.
The JBL 7 balls are primarily intended for root feeding plants and supply the plants with nutrients via their roots. Using JBL Ferropol to fertilise via the water column is generally advisable, as all plants (the stem plants more than the root feeders) also absorb nutrients through their leaves.