Phosphates in fertilizing tablets - Plants and medications - PMDD

I while ago I posted a few times concerning the high phosphates content in
my tank.
I now have put the problem under better control, although things should
still improve. By removing peat and making frequent water changes I reduced
the phosphates content to less than a third of the original concentration,
and I can see a major improvement in terms of slime algae growth.
Unfortunately, I am experiencing now an outbreak of parasitic disease in the
tank, and I had to reduce the schedule of water changes to allow some
medications to (hopefully) have their effect.
For everyone's information, I used formaline and Fluke-tabs (metriphonate
plus I guess mebendazole) without any apparent harm to plants, except for
the dust of eccipients (easy to siphon) coming from the Fluke-tabs tablets.

As many experts stated, the Delaware Imports fertiliser tablets tested
positive for phosphates: I dissolved one in a gallon of water, and the PO4
levels were >5 mg/l. Since my kit (HACH) has an high range option, if anyone
is interested I can verify the levels up to 50 mg/l. Let me know. I guess
they should still be OK if buried deep in the substrate.

I also started using a modified PMDD: I have not added K nitrate since my
NO3 levels are around 8 mg/l (N-nitrogen), I used 1 ml of PH down instead of
HCl (no mold developing yet) and I used muriate of potash instead of sulfate
of K, which I can't find yet. No iron added yet, because the levels in the
tank are still around 0.4 mg/l, and I want first to bring them down before I
add more. I suspect it leaches from the laterite in the substrate. It would
be nice to add the other trace elements, but I do not know how to do so
without adding extra iron. Maybe this eccess of iron shouldn't concern me?
The plants seem to do slightly better, although some remain a little
stunted. Maybe I have just to persist, and adjust the PMDD composition until
I find what fits my tank. If I do not see results, I will first add the
trace elements (with more iron if I do not find an alternative without it),
and then try also with K Nitrate, as Kevin said.