Lets suppose we shoot for 6 months.
The reality of the situation is that your puffers could be 1/2 the size of your fist by then.
If you were to stick both hands in the tank at the same time, that gives you a rough gauge to go by on swimming room.
Forget aggression and forget hormones, that is just swimming room. Add those 2 other factors in and you have more than a slight problem.
Now, if possible, (I'll not do a walk through, because you are an intelligent person and can research products) I'd recommend for your current set up:
2 HOB filters paired together. Not one at one end of the tank and another at the opposite, the fish will look like popcorn in an air popper.
One filter being Mechanical Media (sponge etc..) and the rest Biomedia (
sinking bio balls, pumice stone etc) Don't use the black carbon that came with the filter.. waste of space.
The second filter should contain ONLY biomedia (sinking bio balls or pumice). The choice of biomedia is yours but, make sure you do some research.
Get a media sock and fill that with 1/3 more bio media.
Daily water changes of 25% twice a day (that equals 40% once daily) or 50% daily. Gravel Vac 2 x weekly.
When cleaning the filter, only clean in dirty tank water, never tap water. Chlorine will kill the beneficial bacteria.
Feeding is light and watch them feed. ANY left over food MUST be taken out immediately.
That may or may not keep you for 6 months. Hard to tell.
Also yes, liquid test kits are far superior to test strips. You'll need hard numbers and your LFS can provide these.
Keep an eye out for cheap tanks, craigslist, and (petco?) sometimes have a $1 per gallon sale. That is your target. Once you can get one, you can move EVERYTHING over to the new tank (s). That includes filters, media and substrate.
When watching the fish:
Ammonia poisoning: discoloration, fin rot, blindness and sudden death (can be as little as 3 hours on an over stocked tank).
Nitrite poisoning: blindness, listlessness, bumping into things, lack of appetite. Death within 1 week.
Some will agree, some will disagree, some will add to this (as I have not covered everything), some will say BS...
If I were in your situation.. those would be the procedures I would use.
And ask questions