Yes, agree with Andy there,
You are preparing to do a fishless cycle in the 200L tank and to do a fish-in cycle in the 64 liter tank. Each type of cycle is a completely different thing, so you have to read and follow different rules for each. If its possible that your LFS would take back the fish from the 64L, then I'd strongly recommend doing that so you can just fishless cycle both of them.
Our working articles for how to do these cycles are in the Beginner's Resource Center at the top of this forum.
Why cycle a tank?
When fish move water through their gills to remove oxygen, they not only leave CO2 in the water, they also give off ammonia. The organic waste of fish, dead plant materials and excess food also stay in the tank and are converted by heterotrophic bacteria into ammonia. So as soon as a there's anything much in a tank, there starts to be ammonia.
In the wild, ammonia is diluted and carried away from the fish by millions of gallons of fresh water. Ammonia, even in very small concentrations, causes permanent gill damage or kills the fish. These toxic concentrations appear quickly in aquariums.
Aquarists, for years and years, have been able to take advantage of a really amazing bit of biochemical technology called a "biofilter." The biofiltration takes place inside the aquarium filter. Its a whole separate process from "mechanical" filtration which removes particles and is the only function most people think a filter is for. To make a biofilter we grow two specific species of bacteria by providing them with food (the ammonia!) and oxygen (the fresh water being pumped through.) These two species grow slowly. It usually takes a month or two of the right conditions to grow them. The filter is not ready to handle fish until this process is completed.
The first species eats the ammonia and creates nitrite(NO2.) The second bacterial species eats the nitrites(NO2) and produces nitrate(NO3), which is left in the water.
Unfortunately, nitrite(NO2) is just as deadly as ammonia to the fish. Nitrite(NO2) in even small amounts, attaches to the fish blood hemoglobin and renders it incapable of carrying oxygen. The immediate effect of this is permanent nerve damage and further exposure causes death. Fortunately, our second species of bacteria turns this nitrite(NO2) into nitrate(NO3) and that is much, much less toxic and can be removed with the weekly water change. Nitrate(NO3) is not a great thing to have in your tank but it is not deadly to fish like the other two things.
The members here in our "New to the Hobby" forum are well practiced at helping people get through the process of learning how to cycle. With fishless cycling you use pure household ammonia to grow the bacterial populations prior to introducing fish. When there's been a lack of information and beginners end up with fish in an uncycled tank, there's a sort of emergency method called "fish-in" cycling which involves daily or more large water changes and testing to determine the toxin levels.
Good luck Danny, the members here will try to help you get it all worked out!
~~waterdrop~~
