It doesn't need to be very strong when mixing a bleach dip for your aquarium plants. A ratio of 1:19 or 1 part bleach to 19 parts water is sufficient. Hardy plants can tolerate up to 5 minutes in a bleach solution, but delicate plants should only be submerged for 1-2 minutes max. You will need to prepare a strong neutralizing rinse with a concentrated de-chlorinator to remove any bleach deposits and stop any damage from occurring.