Plant the tank and never worry about a gravel vac again.
Plant the tank, and you will provide hiding spaces so you never need to seclude your fry ever again.
If you prefer to vac, just know that depending on frequency, you could do more harm than good.
Whoever said that the beneficial bacteria are mostly in the filter needs to do more research. It is proven that these bacteria thrive on surface areas. One of the benefits of sand is it has a higher surface area. More beneficial bacteria. I have found a mixture of substrates is best.
The cory argument is moot. I have corys in sand, in gravel and even in colorful sharp rock gravel. Their barbels are just fine. They thrive well in whatever environment they are put in. The only time I have seen barbel loss is due to disease. No joke here. I have read in a lot of places about that. Everyone seems to thing these are dumb fish or something. They still dig through the substrate, no matter what it is. I promise you that.
Has anyone every heard of seeding a tank? You ask a friend, or your lfs, for a cup of substrate from deep on the bottom. There is a reason for this. That is where your beneficial bacteria thrive. Your filter surfaces maybe hold some, and if you have a bio wheel setup, that will form some on the wheels, but your gravel/rock/sand is where the bulk live. They populate the surface area. Some gravels are specifically porous as to allow more surface area for bacteria to take hold. Insisting the filter is the main home of these guys is misinformation. There are no "filters" in nature. The bottom is the filter. The plants are the filter. The organisms in the water column are the filter. The entire eco environment is the filter. Putting a filter on a tank is more for mechanical removal of debris in the water, as well as providing some water support. Chemical removers such as carbon and zeolite, amongst others, will help in situations where you do not have that eco environment going (like my 75 gallon), but these also remove minerals from the water. This causes some diseases in fish as well.
Build the eco system, and most of what is argued can go away.
Be a water keeper and your fish will thrive, no matter what they are.
There are so many disagreements on this forum about substrate, temp, water changes, setups, plants ect...look....everyone has experiences in aquaria. Is everyone correct? No. But there are some misconceptions being floated around that can potentially be harmful. Like the topic here. Gravel vac'ing. Understand that by doing this, you will indeed disturb your cycle. You will remove bacteria. However, in an unplanted tank especially, you risk a rapid buildup of detritus. This is due to forgetting how fish are supposed to live in the environment. They are not fed by humans all day. They have to search and hunt for food daily. Fish should be fed sparingly. Especially in an unplanted aquarium. This cuts down on fish waste, and excess food, which gives the bacteria the ability to keep up. One feeding a day, and let them find what they missed. In a planted tank, the plants will utilize what they can, and there really is no need to vac. You still feed once a.day, but use a variety so the fish have to search and hunt their food.
I do see a lot of overthinking this hobby. Not only here, but all over the internet. Are my ideas the best? No. We saw why last week. But that was not the ideas fault. That was mine. I got lazy and 2 of my fish paid for it. Had I kept on, they would still be swimming away. Life lesson learned. The point to this is, go online and research the environment the fish you plan to kepp.come from. Whether it's sand, gravel, rock, and recreate that environment. You can add some cool attractions, but you also want to create a self sustaining environment that will.require the same amount of care as the place they come from in the first place.
