hmm... Interesting.
I searched for some online reviews of the product and they've all be very good.
Eco-Complete Substrate Review • Land of Fish
https://www.plantedtank.net/forums/29-substrate/147463-eco-complete.html
of course I'm talking about EC for planted tanks. I'm not gonna spend a lot of money on testing the soil by a chemist, but the label on the back claims that it's very nutrient.
what would you recommend? I have a 65G tank which I'm planning to recycle it in May after this tank (20G) is successful and stable.
While the reviews are likely mostly positive, that doesn't change the fact that the substrate is inert. It contains very few nutrients for plants, only a small, trace amount of micro nutrients which plants need very little of. The material is made up of these compounds - but it does not break down meaning the plants cannot utilize them. They stay completely in tact within the aquarium, and are completely unavailable to the plant.
Now, the bacteria packets are of good benefit, but are not necessary. So, back to my original point. You are driving plant growth quite hard / fast with higher light and CO2. With what little nutrients you are supplying them, the plants cannot keep up. They are starving of virtually all 14 nutrients required for proper, healthy growth.
Fortunately, we can easily supply all of these for very little money.
KNO3
KH2PO4
K2SO4
CSM+B
DTPA Iron 11% (if your pH is above 6.5).
That will cover everything you need. Depending on your tap water, you may want to boost general hardness (Calcium + Magnesium sum). This is also very cheap.
MgSO4 - Epsom salt
CaSO4 - Gypsum
Research EI dosing, PPS-Pro dosing, gH boosting for some good information.
Or, I can recommend some successful levels that many planted tankers are using with great success.
For example, with a 50-70% weekly water change:
NO3 - Dose KNO3 to reach 25ppm
PO4 - Dose KH2PO4 to reach 6ppm
K - Dose K2SO4 to reach 25ppm (KH2PO4 and KNO3 will contribute to this as well)
CSM+B (with additional DTPA 11% Iron if needed) Target 0.2ppm Fe 3 x weekly - Say Mon, Wed, Fri. for example
CaSO4 - Target 15ppm Ca above tapwater readings
MgSO4 - Target 7.5ppm Mg above tapwater readings
Dosing all of this is easy, cheap and will cover all your bases. You can build up some root tabs, do not buy Flourish,build your own with Osmocote+ and size 00 gel capsules. Place under larger rooted plants every 2-3 months, or as needed.
Now, if you are setting up a new tank, I would use come black diamond brand blasting sand, or pool filter sand if you want a light color. Rinse it well, and use enough for 2-3" of depth across the tank. More if you want some slopes. You can mix in your current Eco-Complete if you want a more textured look. Sand is 1000x's easier to plant into than eco-complete, the two sands I recommended are large enough to not be easily disturbed, and they are nice and clean to work with.
For sake of transparency, here is my old 65 gallon tank using Eco-Complete: