Hi Robert,
Welcome to Simply and and best wishes for your start in the discus-keeping hobby.
In order to answer your questions in the clearest manner possible for me, let us start off by setting a grading approach to starting off with discus in the safest manner possible, with a minimum of risk-taking:
For example, as a newcomer to discus, if you were to start off with a 125 gal bare-bottom tank to house 8-10 adult or near adult discus obtained from a proven source for supplying good quality fish, and do large daily wcs and frequent tank cleansing of all wastes, at least until you gained some familiarity with discus traits & behaviors , then you would likely qualify for a grading of 9 or 10 on a 1 to 10 scale, ten being the safest, least risky approach. Your success rate in this case would be very high.
If you were to place 10 small young juvenile discus of say under 3" obtained from questionable source(s) in the same-sized heavily planted tank which contained a dirt-type or gravel substrate , had other tank-mates with them, and you did only smallish weekly wcs, with a more or less non-existent tank cleansing routine, then your risks would be very high, putting you @ a true '0' on that 1 to 10 scale. No chances for success here.
In your case, you're planning on:
- A heavily planted environment using an unknown substrate;
- Max. twice a week wcs of unknown quantity (even with 5" or larger fish);
- An apparent minimum tank cleansing routine, counting on many plants taking up most if not all of the wastes being produced;
- Starting with a high tech set-up w/daily use of pressurized CO2;
- And immediately adding other tank-mates, i.e. Congo Tetras (questionable compatible discus tank-mates, Cardinals (ok as tank-mates), and Cherry Shrimp (nothing but quick meals for large discus);
Given these factors, I would rate your overall safety factor and your chances for success at not more than 3 or 4 on that 1 to 10 scale.
Hope this gives you an idea where the problems lie in your case.
If you start with 5" near adult fish (hopefully obtained from a fully reliable source for supplying quality fish), you needn't be concerned with potential stunting, - you should be ok there even with only twice weekly wcs, but that's not all the problem of course.
Your plan to keep them in a heavily planted environment using CO2 from the outset, limited, if any, tank cleansing in who knows what type of substrate, and with other tank-mates, does not lend itself to keeping the tank as free as possible from the development of potentially harmful bacteria, and should not at all be expected to produce good water quality & conditions that discus must have, primarily in the early stages of a new discus hobbyist's experience.
Here are a few suggestions that should make your discus program a safe and successful venture, at least amount of risk to the fish:
First, your pH situation seems quite OK as you describe it, and starting off with say, 2 AC 110 filters, or 2 or 3 large sponge filters should be just fine. Feeding large fish only twice daily is also a-ok.
If you obtain 8-10 - 5" discus from a high quality source, all at the same time, and while doing so, also get a seeded sponge filter from that same source to use from the outset, that should be an acceptable approach when adding all the fish to the tank at once - needing only for you to daily monitor for any presence of ammonia, and dealing with same through large wcs whenever necessary. (see added comments below about starting off with a freshly cleaned-up tank.)
- Suggest you start off with only a lightly planted environment using say, pool filter sand ( at least for the first few months until you become familiar with all aspects of discus-keeping).
- Maintain a minimum of twice weekly wcs of at least 50%, while undertaking a substrate vacuuming and good tank cleansing routine (e.g. glass wipe-downs) with each wc.
Again, at least until you gain good experience with discus-keeping, avoid using CO2 and high-tech lighting, and delay adding any tank-mates for now.
- If you fully clean out that 125 gal tank before your discus start, & do the suggested freshly-done light planting in PFS, then you'll have no need for any quarantining of your newly-acquired discus.
I hope you take my suggestions in the spirit intended, i.e. to propose a successful start-up approach to your discus-keeping.
All the best to you.