How many water changes for 100L tank?

[Tetratank]

Hello we have tank that we recently stocked with 10 neon and 10 lemon tetras. They are doing great.
Just wondering what would be the best maintenance for a tank this size?
We have a weekly cleaner that is supposed to clean the tank, should we stick with weekly 20% gravel clean and water change or do this less often, such as fortnitely? Don’t want to cloud or disrupt the water too much and wondered if there’s such thing as doing too many water changes

Also, we started off feeding once daily but recently switched to feeding twice daily and they seem to mop up both lots no problem with no left over food but don’t want to over feed or have them over poop! but at the same time try to make sure they are all fed

Last question is about plants and might be a bit of a silly question... We have a monthly plant fertiliser, is it safe to put straight into the water with the fish? (It doesn’t say otherwise on the bottle but just thought I’d ask!)

Thanks

For newer tanks especially, a 20% weekly water change is ideal along with gravel vac and to check equipment, such as making sure the filter isn't clogged and the heater is working correctly.

Fish should be fine being fed once daily, it's normal for them to snaffle things up quickly. Over feeding is definitely more risky than underfeeding.

The fertiliser should be fine to go straight in the tank, make sure to follow the instructions on the bottle :)

Al of the above and I would also add to never clean your filter sponges with anything other than the fish tank water, don't even use tap water as it restarts the cycle every time and is not good as you want the good bacteria to build up so it will become more self sufficiant. After about 6 to 8 weeks you can drop the water changes down to 10 percent and a gravel clean once every 2 weeks.

With the feeding once a day is fine fish do not have the off switch telling them they are full like other animals you could stand there all day and feed them and they will eat it but the more you put in the more ammonia they will produce which is bad even fatal for them, what food are you feeding them btw?

Yes the plant fertiliser is fine to use with the fish in there they are designed not to be harmful for them.

How old is the tank and how long did you leave it before putting the fish in, did you add the 20 fish in, in one go? If so I would even drop the feeding down to every other day as it's a lot to add in together before the tank has cycled.