Safe to put in more fish?

[Eoin]

I recently exchanged the plastic plants in my tank for live ones. I didn’t change the substrate or the filter, thought I did dump most of the water in order to facilitate me moving the tank to a new location in my apartment. In the week since setting it up, I haven’t noticed any ammonia levels. I did have nitrite levels of about 0.4, but they dropped to less than 0.1 less than two days later, and are currently so low that I can barely detect any colour.

Several days ago (after the nitrite levels dropped), I put some more fish into the tank. The pet shop employee I spoke to, suggested I only get a few fish at a time to avoid any ammonia spikes. In the three days since putting those fish in my tank, my ammonia levels have remained at 0 and my nitrite levels have remained at the barely visible level. The nitrates have also not budged, and are well within healthy levels.

I’m wondering, as I didn’t detect any ammonia several days after I first changed my tank over, and haven’t detected any after bringing in the new fish, should it be safe to put in the last of my planned fish? Do I need to wait longer for ammonia to concentrate enough to become noticeable, or does the fact that I already have nitrates in the tank indicate my tank has already finished cycling? I know it seems like a fast cycle, but as I mentioned before, I was using old substrate and an old filter.

Should it be safe to put in the last of my fish?

[Keeper]

when you say "the last of your fish". How many is that? You only want to add a few at a time until you are done. I don't reccomend dumping in a large qty of fish for it could cause a large ammonia spike.

[Menagerie]

It depends on how many fish and their current size. How large is the tank?

[Eoin]

The tank is 20 gal. I currently have 6 cardinals (3 of whom are fully grown, the other 3 are almost there), 1 pepper corry, and 2 pygmy gouramis (my recent addition).

By “last of my fish� I mean 4 Black phantom tetras, and 2 more pepper corrys. (plus some form of algae eater later on - haven't decided what I want)

I’m curious, when it is recommended that you not go over 1 inch of fish per gallon of water, is that for space purposes, waste purposes, or both? I realize that I will be just over that number, but will more frequent water changes solve the problem?

[Menagerie]

Space, oxygen utilization and waste production. More frequent water changes certainly will help and having a filter that turns over the water 8-10 times per hour (gph rating). Normally, you want a filter that does 6 times gph, but when you overstock, you want the number higher.
I would add the rest of your fish and keep a watch on the water parameters. It would also be a good idea to QT your fish before adding them to the main tank.