Never rely on advice from a fish shop, too many people learn the hard way that a lot of the worst advice is given by fish store employees, especially from the big box stores. Always do your own research and never just take their word for it.
Plecos need to have access to driftwood at all times, they rasp at the wood and it has been discovered that it's essential to help their digestion. Wood shouldn't turn your tank green, either something else was responsible for that (and new tanks are prone to algae blooms, which turn the water green) or you got a type of driftwood that releases a lot of tannins, which turn the water a tea brown kind of colour. Tannins are helpful for most fish, just not what everyone wants in their tank. There are a lot of driftwood that don't release much in the way of tannins though, and pre-boiling and soaking the wood before adding it to the tank can remove a lot of the tannins too.
We're going to need those water test results I'm afraid when you get home. Which test kit do you use?
The water changes you're doing aren't going to be large enough, especially not if your stock is "lots", even in a large tank. 50 litres is less than 15% of the water volume. Most of us change 50-70% weekly, even in tanks that aren't heavily stocked.
The shop is right that temperature differences when you do a water change will shock and kill your fish. But the answer to that is to temperature match the water you're replacing to the tank water before adding it back in, not to do tiny ineffective water changes. Not forgetting to use a water conditioner either since most tap water has chlorine and chloramines in it.
We do water changes for several reasons. It removes the nitrates produced when your BB convert fish and food waste from ammonia into nitrites, then into nitrates. Nothing else (with the exception of some complicated sciency stuff or difficult methods) removes nitrates from the water except for water changes. If you do a 50% water change, you remove 50% of the nitrates, and for most tanks, that's enough to keep the levels safe. A more highly stocked tank with fake plants, might need more. If you're only removing less than 15% of the nitrates each time, then your levels are going to keep getting gradually higher over time since you're not removing enough to keep a balance.
We also do water changes to keep the tank water chemistry close to our source water chemistry. As water evaporates, minerals and things are let behind, and become more concentrated in the tank. Changing a decent amount of the water and gravel vac-ing the substrate helps keeps those levels in balance too.
In emergency situations like when an illness breaks out, or the tank gets a whole tub of food spilled into it, large water changes are essential to save your fish, as much as 90-100% daily water changes sometimes. If your tank hasn't had large water changes for a long time and the minerals, pH, GH, nitrates etc have drifted so the tank chemistry is very different from your source water, those large changes will kill your fish. So weekly maintenance is essential.
