I have a 29-gallon tank and, as I recall, when I changed out my gravel for Eco, I used 3 and 1/3 bags. (I had the 1/3 bag left over from another tank I had set up, so I just threw it in so as not to waste it.) In addition, I recently set up a 10-gallon, and it took one bag of Eco. So based on my experience I would say you need one bag of Eco for each 10 gallons of tank capacity. However, if you leave your gravel in, you would obviously need less. If you replace the gravel, I estimate you would need at least 7 bags. (If the store will give you your money back for any unused bags, you might want to buy 8 or 9 bags, just to be safe. Whatever you do, don't buy too few bags and discover in the middle of your change-over that you need to make a sudden run to the fish store -- see below.)
As far as changing your substrate out, what you do is get 2 or 3 big plastic tubs and drain your current water into them. (I used a 25-gallon storage tub from Target when I did my 29-gallon.) Put your fish in, along with your decorations, plants, heaters and an air pump for each tub. If you can get your filter(s) to function properly, attach it/them to the tubs as well. If not, be sure to keep your filter media soaking in your used water so your "fish goo" doesn't die. Also, don't worry about having no filtration for a short time. You can change out your substrate pretty quickly once the tank has been drained, so your fish should be able make it.
After you get the tank drained, remove your gravel and put in your Eco. Don't throw the Eco bags away. Instead, after you get the Eco in to the depth you want, lay the Eco bags on top of the substrate and pour your new water in. This will keep cloudiness to a minimum. Once the tank is almost full, take out the Eco bags and set up your filter(s), heater(s) and thermometer(s). Also put in your plants and decorations. Let the tank sit with filter(s) running for about 30 minutes. This will allow some of the cloudiness to clear. In the meantime, check the water parameters on the newly filled tank, especially the pH. Also check the water parameters on the tub water, especially the pH. (Hopefully, the pH's will be relatively close.) Eco has some biological stuff in it, so if you've kept your "fish goo" alive in your filter(s), that together with the Eco "goo" should preclude your having a new cycling phase.
After you've gotten everything changed out and put back together, you can acclimate your fish to the tank again.
I've done this twice -- once on my 29-gallon and once on a 5-gallon. I didn't lose a single fish either time, and I have some fairly sensitive fish, including rummynose and cardinal tetras. Granted, they looked a little faded for a couple of days, but all of them survived.
The trick is to have EVERYTHING you need before you start, including properly heated water! I use RO/DI water in all my tanks, so I had six 5-gallon jugs of it on hand when I did my 29-gallon tank. Turns out, I only used not quite 5 of the jugs of new water due to the displacement caused the substrate, decorations, etc. I also had the new water heated to about 76 degrees, so I didn't have to wait for the water to heat up before I put the fish back in. It took me about 6 or 7 hours to do my 29-gallon, but that included a 2-2.5 hour re-acclimation period due to my "weanie" (meaning, delicate) tetras. It took about 1.5 hours to do my 5-gallon tank.
I did both change overs by myself (and I'm an old bag of 58), so if I can do it, you can do it. Good luck!!
As far as changing your substrate out, what you do is get 2 or 3 big plastic tubs and drain your current water into them. (I used a 25-gallon storage tub from Target when I did my 29-gallon.) Put your fish in, along with your decorations, plants, heaters and an air pump for each tub. If you can get your filter(s) to function properly, attach it/them to the tubs as well. If not, be sure to keep your filter media soaking in your used water so your "fish goo" doesn't die. Also, don't worry about having no filtration for a short time. You can change out your substrate pretty quickly once the tank has been drained, so your fish should be able make it.
After you get the tank drained, remove your gravel and put in your Eco. Don't throw the Eco bags away. Instead, after you get the Eco in to the depth you want, lay the Eco bags on top of the substrate and pour your new water in. This will keep cloudiness to a minimum. Once the tank is almost full, take out the Eco bags and set up your filter(s), heater(s) and thermometer(s). Also put in your plants and decorations. Let the tank sit with filter(s) running for about 30 minutes. This will allow some of the cloudiness to clear. In the meantime, check the water parameters on the newly filled tank, especially the pH. Also check the water parameters on the tub water, especially the pH. (Hopefully, the pH's will be relatively close.) Eco has some biological stuff in it, so if you've kept your "fish goo" alive in your filter(s), that together with the Eco "goo" should preclude your having a new cycling phase.
After you've gotten everything changed out and put back together, you can acclimate your fish to the tank again.
I've done this twice -- once on my 29-gallon and once on a 5-gallon. I didn't lose a single fish either time, and I have some fairly sensitive fish, including rummynose and cardinal tetras. Granted, they looked a little faded for a couple of days, but all of them survived.
The trick is to have EVERYTHING you need before you start, including properly heated water! I use RO/DI water in all my tanks, so I had six 5-gallon jugs of it on hand when I did my 29-gallon tank. Turns out, I only used not quite 5 of the jugs of new water due to the displacement caused the substrate, decorations, etc. I also had the new water heated to about 76 degrees, so I didn't have to wait for the water to heat up before I put the fish back in. It took me about 6 or 7 hours to do my 29-gallon, but that included a 2-2.5 hour re-acclimation period due to my "weanie" (meaning, delicate) tetras. It took about 1.5 hours to do my 5-gallon tank.
I did both change overs by myself (and I'm an old bag of 58), so if I can do it, you can do it. Good luck!!
