Planted Tank Guru
Join Date: Jan 2010
Location: Contra Costa CA
Substrates that cause a minimum of water chemistry issues:
Eco Complete. Occasionally I still hear of a bag that raises the pH of the water. This is rare. I would trust it to be inert in the senses that you are asking about. It will hold nutrients for the plants. (Cationic Exchange Capacity) You do need to add those nutrients, of course.
Pool filter sand. This has no CEC, and is almost always light in color. A bit better than gravel, but not by much.
Pebble Tek and similar materials. Inert, and you can get all sorts of colors. It is very dense, so it will hold plants down, but really is just a glorified sand.
Flourite has a large product line, and it is worth looking into. Several products are highly thought of for planted tanks, high CEC and dense enough to hold the plants down well until they root.
Acceptable water chemistry changes:
If you are keeping soft water fish and do not mind that the substrate can remove the KH, then Safe-T-Sorb and its relatives would be a great choice. You can add fish pretty much right away. Turface (Landscape/ Greenskeeper product) is similar, but I do not like the color choices now available. High CEC, and I like the soft, natural color of Safe-T-Sorb. These materials tend to be a bit light weight, though.
If you are keeping hard water fish and want the benefits of a substrate that raises KH, GH and pH then coral sand is OK, but is light in color.
You know ahead of time that you are moving.
Not all the nitrifying bacteria is going to survive the move, especially when you are going to swap out the substrate at the same time.
Get a 5 gallon bucket or similar tub and do a fishless cycle with any sort of media that will grow bacteria. This will give you a big reserve of nitrifying bacteria that you can add to the tank when you move and set it up. Cheaper than buying bacteria in a bottle, and you know it is working!