While you reading let me share with you some thoughts.
Your data do not make alot of sense to me when it comes to the zero po4 no3.
You moved a 5 years old system
Used rocks that should be saturated with nutrients yet you did not clean it.
Your used the same sand, stirred it, did not rinse and put it in your system
You have massive bio load for your water volume which means lots of feeding
Your water change is 10g only so nothing crazy to justify reduction in po4 no3 that much...
All that,yet withen 3 months you went through a cycle and your nutrient levels are zero. You do not have cyano, gha or distoms...its hard to think 3 months is enough for the system to recover from all its nutrients.
Are you sure your testing is correct? Your kits are accurate? How much do you feed? What's your husband's like? Do you do anything during water change other than water change itself?
Yes we need to correct your parameters but this actually is not a big deal and can be done in a week or 2, but what I mentioned above is worrying me more. If we have high nutrients but we are unaware we will get results that are unexpected..
Either your test is wrong
Or indeed your levels are so low which might mean you are still in the cycle
Or previous owner used carbon dosing that is still in your system.
Any of these points ring a bell?