As long as you do not have a nitrate test, it would be safe to do bigger and more frequent waterchanges.
I do a 50% waterchange weekly, combined with gravelvacuuming.
Only way to tell for sure how much and how often, is to get a nitrate test.
A liquid testset would be your best option and will be more accurate than teststrips.
I try to keep my nitrates below 40 at all times.
As for the snails, i wish i lived near you.
I can never have enough snails for my puffers!
Usually overfeeding a tank will make snail reproduce very fast.
If you want to catch the snails, take a slice of zuchinni, and drop that into the tank.
Leave it in there overnight and in the morning a lot of snails will be on the zuchinni.
Just take the zuchinni and snails out and repeat the process.
Keep in mind that if you have a snail infestation, they will most likely return in time.
Other option is getting loaches, for they eat snails, but as your tank is only 50Gl, you do not want to add anything to your bioload.
What may work, in addition to catching them with a net or zuchinni, is buying some helena snails.
These are known as "Assasin-snail" or snail-eating snails.
they will add some to your bioload, but will be a great help in snail-control.
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