The gravel vac will really help. By changing all of your water, you risk disrupting the cycle of the tank. Smaller changes are better for that. Anywhere between 10 and 20% should be good. Also, your tank is WAY over stocked. That is probably why the waste builds up so quickly. A good rule of thumb for beginners is about an inch of slim bodied fish per gallon. And snails, depending on the species, need 5 to 10 (or more!) gallons, per snail.
So, the calculations to get your minimum tank are:
cories, (not slim bodied, so allow extra...we'll say two gallons per inch). Most species of cories get about 3".
so for each cory, allow 6 gallons, you have 3, so 18 gallons.
female betta, 2.5 to 3 gallons, (subtotal, 18+3=21 gallons)
guppies, 2.5" each, times 4= 10 (subtotal, 21+10 = 31 gallons)
plus the snail (31+5=36)
So according to that rule, your minimum tank should be 36 gallons. Not sure you really need that much, but you definately have WAY too much in there. Some say a well-stocked tank may look empty. It is better for the fish for many reasons to not be over crowded. I suggest getting a 2nd 10 and putting the cories, and snail in it. They are more peaceful and relaxed than the others.
Hope this helps
amber