To start off with any good aquarium book about live plants will advise you to use a 1-3mm natural river pebbles/gravel
and a minium depth of 3ins.

As far as I know there is no such thing as a perminent substrate it must be cleaned adventually I have let a well planted tank go for about 10 years I had a UGF and a full back drop filter.

Sand can be a big problem for many plants as the fine particles of the sand will not allow the roots to breath and can actually crush the roots. It also can cause many other problems especially with filters.

Keith

Hello
I'm no expert lol but would have to agree with Keith.
Any substrate small enough to fit in an aquarium is eventually going to fail. Growth is usually the main problem
Naturally as it grows it consumes the available nutrients as well as the CO2.Certainly can with a lot of tinkering, last a long time but not forever lol.
Just setup a 150 using 3 inches of top soil with two inches of fine gravel on top. with natural sunlight . I'm getting fantastic growth with rooted plants but not floaters. Of course , I'm using 1200 gph circulation which would explain that.lol
IMO a "permanent" setup would probably have to be at least 1000 gallons with a purge system. Even then ,I suspect there would still be maintenence but not nearly as much. i have a 5x15 foot pool that works very well but without constant removal it would soon choke out. I keep all plants in pots to make rearragement as well as spot fertilizing possible.
Also keep a 75 gallon paludarium almost stationary but with mostly terrestrial plants. Using pure silica sand. Does require extensive water changes though.
gary

180 gallon is a BIG project for a planted tank but do-able.
I had a 50 gallon tank and now upgraded to a 60 gallon, in my new tank a used a mix of shultz aquatic soil, latrite & Florite. I have had great growth in my tank and think that it is great combo. The Florite is a bit pricey and for a 180 gallons WoW$$.

About cleaning your tank I clean the top of the gravel only. I wave my hand over the plants to stir up the mulm (poo) and suck up the water around the plants.

You are correct that the plants do use the nutrient in the gravel but you always need to do your water changes. In the wild the fish always have fresh water from springs and rain ect. Alot of plants take their nutrients from the water column so fertilizing is a must also.

A 50% "Florite" and 50% micro-gravel or 100% "Florite" would be fine. This is really only necessary or heavy root feeders. Most stem/rosette plants take the nutrients from the water column as ed said. You could always just use an inert gravel and supplement with iron ferts also to save money. The fish will take care of adding nutrients to the gravel.
As far a gravel vacuuming, most people with heavily planted tanks just do a skimming of the gravel and decor weekly. Every 8 months (approx) remove all the plants and the fish to a extra tank or fish container and do a thorough gravel vacuum. Re-root and restock. This is just one way of many but after trying different methods this one is the most effective and easiest one for me.

Hey garyfla! So this is where youve been! LOL!