Anytime you add more stock you need to watch the levels and make needed water only changes until the biological filter catches up...usually doesn't take but a few days...all depending on what you added and how mature the system itself...its also a good idea to back off on feeding for the first 24h too...
Sometimes you have to make daily 25-50% "
water only" changes until the biological filter catches up with the bioload....
The beneficial bacteria are sticky and adhere to all the surface areas inside the tank-like the walls, decorations, plants both fake and real, in the top layer of the substrate and in the filter media-very little are in the water column itself and so water only changes will not hurt the cycle-but over cleaning can-you want to limit vacuum to 1 weekly-but at least weekly in all areas that can be reached without moving anything- otherwise the BB can suffocate-wipe down all the viewing walls weekly and best to leave non-viewing walls especially if they have algae since the algae can be helpful for the system
Filter media needs a good swish/rinse in the bucket of old tank water a couple of times a month to get all the big pieces of gunk off-they should look dirty but still have good water flow to keep the BB healthy, thriving and colonizing.....
You don't need to change the filter media unless its falling apart and then you want to cut a piece of the old filter media off and attach it to the new filter media for a week or so to help seed the new media faster.....however, you still want to monitor the prams since you may still get small spikes that need a water only change to correct....
Ammonia and nitrite you want to keep 0.25ppm or less-Ideally 0ppm
Nitrate 5-10ppm ideally and less than 20ppm-
High nitrate is generally tolerated by most species of fish since it is gradual-but a sudden change (
large water change) in that nitrate can shock them and even kill some species of fish-then if you add new fish that came from low nitrate to high nitrate this can cause health issues for them and then the old fish can get sick due to their compromised immune response secondary to living in a high nitrate tank.....
Long term high nitrate (
over 50+ppm) can compromise immune response of the livestock and stunt growth of young fish and sometimes cause the death of shrimp
With high nitrate in the tank-this usually means the tank was neglected- has not been taken care of or poor vacuuming or filter care-with high nitrate usually you will also have low pH-this naturally happens in closed systems due to decomp and neglected tanks-this can cause other issue and where several myths come from "Water change killed my fish" "Fish shop sold me sick fish"....
Anyway...I am sure more info than you wanted.......but its good info to know why some things happen, what you can do and how to prevent...