[MatPat]

Laterite will provide a source of iron and some micro nutrients but very little of the macros (N,P,and K) that the plants need the most. This may work in a low light, non CO2 setup but since you plan on using CO2 you will more than likely need to supplement the macros.

Why do you think you need to replace the inert substrate and what substrate are you using? There are a lot of people using inert sand and getting great results by simply fertilizing the water column. I've used 3M's Color Quartz for a few years now and love it.

Personally, I think 2" of Laterite is a bit much and probably unnecessary. I had an unfortunate experience with Laterite when I first started out with planted tanks and will never use it again. It leached into the water column through my substrate and kept my tank a nice orange color for several weeks. I eventually had to tear down the tank and replace the substrate. If you do decide to use Laterite, make sure you place enough substrate on top of the Laterite to cap it well.

If you really want to use a nutrient rich substrate, take a look at Aaron's Mineralized Substrate article in the Library. I think this will give you much better results than using Laterite alone.

 

[MatPat]

I'm not sure where you got your information from about not having good results inert substrates but it is incorrect A lot of people have great success with inert substrates but they usually require the addition of fertilizers at least every other day. If you cannot dose your tank daily (or every other day), I would definitely look into Aaron's Mineralized Substrate article. I do not think Aaron has to add fertilizers to his tank and states in the article he only occasionally has to add potassium.

Your plants not doing well could be the result of several things, with light being the biggest factor. How much light do you have over your tank and what type of light (normal fluorescent, Compact fluorescent, T-5 High Output, etc) is it? Light is the main factor in plant growth followed by CO2, nitrogen, potassium and phosphate. If any of these are deficient, plant growth will slow or even stop. The plants need to be fed too even though a lot of people have success without using CO2

Flourish and Flourish Excel probably will not provide enough macro nutrients (Nitrates, Phosphates, and Potassium) for your plants but of course this depends greatly on your light level. Most people with high light, CO2 injected tanks use Flourish as a "micro" fertilizer since it contains small amounts of N and P that are simply too low for a high light, CO2 injected tank. I too have extremely hard water with a pH of 8.4-8.6 from the tap. However, the pH drops to 7.6 after it has rested for 24-36 hours or after I aerate it for a few hours. The "rested" pH is a more accurate reading than the water straight from the tap.

High pH and inert substrates can work but I think Aaron's method would be a good start for you. I think the hardest part of using a "Mineralized Substrate" is the preparation of the substrate itself followed by being able to slowly fill the tank so the substrate is not disturbed. Once the tank is filled it should be smooth sailing for a couple of years.