I'm new to planted tanks. Haven't had an aquarium of any kind for 45 years. Kids got me a terrarium and I saw aquascaping videos and couldn't resist.
In My first tank I used black diamond worm casting potting mix capped with black aquarium sand. The plants are(ludwigia, Amazon swords, alternanthera) growing like crazy. During the cycling process I used the API master kit and did the rookie thing and didn't shake the nitrate bottle and just blasted the tank with kno3 and may have caused my plants some stress.
I'm starting a second 40 breeder (as is the first). The only thing that I'm noticing on the first tank, is unless I do weekly water changes the phosphate goes thru roof. I'm not dosing any phosphate and only csm+b and a small dose of kno3. I'm wandering if the phosphate is coming from the worm casting mix. I'm conflicted on whether or not I should use the worm casting mix again or not. The plants are growing almost out of control on the first tank. Water I'm adding test low for phosphate. Should I mix 50/50 with topsoil or just go with what I started my first tank with? Only about 15 tetras.

How deep a layer did you do on 1st one, that’s some very rich stuff. Would probably only need 1/3” at most.

It probably averages about 1.5 inches. Didn't have any problems with cycling. Ammonia and nitrite zeroed out after only a couple of weeks. I know some have said don't worry about phosphates. It is really the only glitch in this setup. Cause it seems to read off the card if I don't do water changes every week. Nitrate I can manage with one dose a week, keeping about 10 to 20ppm. I've read allot of the discussions concerning soil verses inert and for a beginner i feel the soil has treated me fairly well. Except for the phosphate. The only thing I suspect to be raising the phosphate is something in the soil.
I'm using a canister filter and I clean it every month or so. Using tank water. Maybe I'm worrying about the phosphate levels more then i should.

I am not sure how others worry about phosphates, but in lakes and streams the key nutrient that triggers algae blooms is phosphate, and to a lesser extent nitrates. That is why some communities and counties have banned its overall use in landscaping. This is owing to runoff causing lakes in particular to become eutropic https://www.usgs.gov/media/images/highly-eutrophic-lake.

While saying that I am setting up a screened compost 1-1.25" (sifted through screen) and clay 20L with 1-1.5" sand cap. So I am also interested in what people think.