The tetra brand is readily accepted by most O's, however it is lacking nutritionally. New Life Spectrum is absolutely perfect nutritionally, but most O's that I have heard about ,including mine, don't like them. I have bypassed my O's taste buds by packing crickets chock full of soaked NLS, and she can't tell that it's even in them.
I totally agree with everyone else as far water quality. You can not get away with the same maintenance regimen with large fish as you can with the small community fish. The "I work in a pet store so I know what I am doing" is wrong. Taking care of fish for a few days while they are waiting to be sold is a whole different thing than taking care of that same fish over the span of it's entire life and meeting ALL of the many needs that arise, as far as water quality, food requirements, tank size, enrichment, etc.
I used to use the dip strips, until one read 5ppm ammonia. I went ahead and did a 95% water change, just to be safe, but I saved some of the old water and bought the API master kit, and re-tested. It was zero. It is not an overstatement to say these test strips are dangerously inaccurate.
I am not trying to jump down your throat, but all of us on this site are Oscar lovers and some of us(me) have fought hellacious battles to save our babies lives and have truly fallen in love with our little blighters and want the best for all O's, and if we can stop someone from maybe making the same mistakes we did, well, we accomplished what we joined for.