The bacteria colony will only ever be large enough to support the tank's current stock.
So if you take the sponge filter from a tank holding only a few fishes or shrimp's,then you cannot safely add a bunch of fishes/shrimps that far outnumber those that were in donor tank from which you borrowed the filter material.
All you are really doing is transferring some bacteria which saves you from waiting week's to establish it otherwise.
The bacteria are fairly quick at multiplying which is a good thing, but is waiting game for it to initially establish without the use of borrowed media from already established or cycled tank as some like to say.
I do not much care for the term "Cycled" for it implies that at some point the process is done when in my view ,,it simply means the tank is maturing or matured.
The bacterial colony grows and or decreases as is needed for the amount of organic matter present.
Add a few more fish (within reason),feed a bit more food as a result,and more bacteria develop for increased organic matter food's,poop,decaying plant matter, etc.
Remove some fishes,and bacterial colony will adjust to only what is needed.
With lot's of plant's which readily take up ammonia,nitrates, + borrowed filter media from already established tank,one can more easily add a few small fish at a time with a week to ten day's between more addition's.
The tank in question 7.5 gal will likely be closer to five gallons with substrate and hardscape so would not plan on adding much in the way of fishes.
Would be interesting shrimp tank.
