Can it be the QT tank for starters since there is nothing in their for them to infect?
If i understand this right, your asking if your main tank has no fish in it, can it be used to QT
the first batch of fish??
While it is possible i would reccomend against it, only because say the fish have a disease such as ich, it will multiply in the tank and spread, and one of the common treatments is copper, now if you add copper to it , the whole tank has copper and you might never get to add any inverts to it because copper stays in for a long long time.
Another downside, is if the fish do have a disease you have to treat way more water then if you put them in a small QT
tank, which means way more money, i've learned from experience trying to treat ich, the QT
tank is the better method by far.
And what if your filtration system doesent have a sump.
A sump isn't necessary for all tanks. I'm not sure what you were asking, if you add some more details id be glad to help.