I suppose everyone has their own experience and preferences and there are many roads to success.
Sand has pretty much zero CEC, but I've used it a couple of times and got pretty good growth with just water column dosing. Strangely enough though my stems didn't do very well. I guess over time sand will become infused with fish mulm and rotting plant material and will form a soil of sorts, but if I use sand it's usually for cometic reasons first, so any DOM in sand is not always desirable. Either way, if it eventually turns in to a substrate with similar properties to soil, why not use soil from the outset; no need to wait, and probably less mess.
I've also grown plants in AS powder and AS and haven't really noticed any difference in growth. The only advantages I can think of is that it's perhaps a little easier to plant small carpet plants with little or no roots in powder, and it provides a better sense of scale in nanos. But I tend to part bury my plants in the substrate anyway so they don't come loose. They soon grow new leaves and start to spread either way.
Like I've mentioned before, I prefer soil substrates, either Gucci or regular dirt. Plants can uptake nutrients through roots and leaves etc so it just makes good horticultural sense to feed both sites. There are other benefits as well...buffering against mistakes like missed fertz doses, and deficiencies, and perhaps greater microbial activity which may infer greater stability, and quicker.
I also like the pH and KH buffering capacity of AS as well since I've always lived in hard water high pH areas. Over all I think soil allows for more wriggle room, which is always helpful especially when you're just starting out.
Plain old sand capped dirt works just as well as AS and doesn't need to be messy either if a soil retainer is used. However, Gucci substrate is very convenient and easy to use, especially if you're a professional conducting a workshop. But don't forget professional aquascapers are also selling product, so of course they are going to use whichever Gucci product the sponsor or shop require them to.
But back to the original questions...
1) Can anyone advise on a good substrate for a small 25L Thailand style betta planted tank?
Personally, I'd use ADA AS or Tropica AS. Normal or powder will work well, but if you want to maintain a sense of scale use powder; smaller grain size.
2) For a bigger tank I would like to have with sand, does anyone know if there is a good substrate that is suitable for going underneath with sand on top that won't rise to the top?
Yes a
1:1 mix of aquatic compost and peat, or
Tropica Plant Growth Substrate. Compost/peat is cheaper and pretty much the same thing.
3) Is there any substrate for plants that would be suitable to use in a BiOrb on top of their ceramic biological media? Something that won't scratch acrylic, will let plants take root, won't clog up the filter and biological media. Don't hate me
Tall order I know.
Not that I'm aware of, it's likely to clog up the filter. I guess you could perhaps use small pots?
Check out Dennis Wong's video on substrates...
