it's not an over load of the bio system. that would lead to a brown, hairy algae like substance called diatoms. since you don't have that, your bio system is fine. the nerites, like shrimp, will only increase your bio load if their numbers are high, as in the tens and hundreds. i have discussed this at length with a guru buddy of mine over last night and this morning, and what we came up with was this. if the frogs seem healthy, and you are not having any problems with the tank otherwise. leave it alone for a couple weeks and see how it does. i'll be honest, in the 30+ years of keeping tanks, i rarely ever test my water. unless it is a sensitive species such as crystal red or sulawesi shrimp, i always used my flora and fauna as my indicators of water quality. i learned long ago that the more you fuss over it, the more problems you seem to have.

my only other suggestion would be is to cut some of the water change water with distilled (or if you have access to it, RO water) water. since it is void of minerals, it may give you readings that you prefer.