"is the filter working" water tests are only going to answer one thing, and if you've had fish living in it for a year, then I'm gonna venture to guess the answer is yes, the filter is working, however I always tend to doubt 0 Nitrate readings, but that's irrelevant.
Ever heard of TDS shock? IF you read up on PH shock it usually spills into TDS shock. Long story short you can have 2 different water sources that both test the same when you do the typical Ammo, Nitrite, Nitrate, even PH tests, but yet test wayyyyy different in PPM (TDS) or Microsiemens (conductivity). IF you have a probe then you know why the fish died. IF you don't, you call it a fluke, unexplainable death, or just bad luck, then the insanity starts...... Doing the same thing over and over again expecting different results.
Most people here don't seem to acknowledge that testing water parameters involves more then "is the filter working" tests. My personal experience is regretting not buying conductivity and PH probes years ago. Measuring conductivity can really change how a person views "clean" water. IF you're really going to keep expensive monsters for a long time, I'd highly recommend doing a little research into some good testing equipment, cause I speak from experience when I say this----- You can feel really stupid when you spend LOTS of money on fish year after year when you really have no clue about the water you're keeping them in.
Don't feel bad if you don't want to acknowledge the fact that getting a good idea on water chemistry may costs hundreds of dollars, don't worry.... The masses believe that a fish will adapt to your water no matter how far from their natural habitat it is, so you can just assume everything is fine. It's cheaper and funner this way. After all it is just a "hobby".......