I have experience with both plain sand (also plain gravel) and Flourite. Flourite and Eco-Complete are basically the same thing, so I see no reason to doubt that my experience with Flourite would be any different with E-C.
I have had Flourite Black (and on its own, not mixed with anything) in my 70g for over 15 months. The plants are not better than the same plant species in my sand and gravel substrate tanks, and all 7 tanks receive the same fertilization and have the same light, and water changes are 50% weekly. So the comparison is on an even basis.
If you still have to dose fertilizers the same, there is no benefit to Flourite or Eco-Complete, but there are downs:
- The cost is astronomical--in my case $180 for the 70g, compared to $14 for the even larger 115g with sand.
- Substrate fish have problems over F/EC. I had to remove my corys.
- There is no nutritive value to F/EC that I noticed.
I am moving over all of my tanks to sand, the 70g is next. I may leave the 90g with fine gravel, as it is doing well and the aquascape resembling a South Asian stream is nice and appropriate to the loaches.
I myself would never consider "prep" or a few days cloudiness as reasons to not use a substrate which otherwise had so many benefits. I had several weeks of non-clarity in my Flourite tank too. But in both this and plain sand/gravel tanks, this cloudiness is most likely a bacterial bloom and can't be escaped. Why it doesn't always occur i don't know.