How to Safely Replace Aquarium Gravel
Whether you want a new look for your aquarium or a healthy alternative for your fish, gravel replacement can be tricky. While the most important beneficial bacteria live in the filter, plenty of ammonia- and nitrite-eating organisms colonize on various surfaces, contributing to the nitrogen cycle. If your tank has been set up for more than a few months, a good portion of your bacteria live in your gravel, and removing it altogether will overwhelm the nitrogen cycle, resulting in ammonia and nitrite spikes that can harm and kill your fish.
There are safe procedures to changing out gravel. It just takes patience and preparation.
Option 1: Temporary Setup
There is one way to do a complete gravel removal in a day and not harm your fish, and that’s setting up another tank for the new substrate. Basically, you are running a fishless cycle on another tank full of gravel; this means you will need a filter and ammonia to start up the cycle and dose occasionally.
Run this fishless cycle until the readings read zero ammonia, zero nitrite, and any amount of nitrate. This will probably take up to a month to accomplish. Once the tank has cycled, spend the day removing the old gravel and replacing with the newly colonized gravel.
Pros: You can keep your fish tank looking great and then instantly change the gravel once the temporary setup has cycled.
Cons: Setting up another tank can be a hassle and costly if you need to buy the equipment, not to mention a month can be a long time for some people to wait.
Option 2: Seeding Gravel
Take some pantyhose (60 cents for a pair at Walmart) or filter media bags and fill them to the brim with new gravel. Then place these sacks in the aquarium after thoroughly rinsing them. After a month, remove the old gravel and pour in new, rinsed gravel along with the cultured ones in the tank.
Pros: Pretty simple and cheap.
Cons: You are only colonizing a small portion of your new gravel. Colonizing at least half of the gravel would be significant, but this would mean overcrowding your tanks with bags of gravel.
Option 3: Replacing Sections
This option is my favorite. I’ve seen videos of people using dividers for tanks (like betta dividers) to separate the gravel so you can replace a portion at a time. This way you have most (or half) of the gravel's beneficial bacteria throughout the procedure.
Chain pet stores and local fish stores (LFS) will carry these dividers, but it’s cheaper to make your own, using plastic mesh found in craft stores, suctions cups, and in my case, plastic needles. Make sure to secure the bottom with suctions cups as well as the sides. Might as well cut the divider a few inches above gravel so fish still have full room to swim. Just make sure the top of the divider is smooth so it doesn’t catch fins.
If dividing in halves, wait a month before changing out the other side. In smaller tanks, you can just push all the old gravel to one side, divide, and place in new gravel. For the first four weeks, you’re not going to lose any bacteria.
Pros: You’re colonizing a large portion of the new gravel without setting up another tank.
Cons: Might look strange if changing out the color.
Option 4: Any Combination of the Three Methods
No matter which method you use, be sure not to take out any filter media during this process, even chemical media. I would wait at least two weeks after partially changing out media to begin this process, and wait a few weeks after all gravel has been replaced to remove anything from the filter. So basically, don't remove any media for six to eight weeks. To avoid clogging up the filter with debris over time, take out media at least once during this process and carefully rinse with aquarium water (because chlorine from the tap will kill bacteria unless treated).