Welcome to TFF, SJM!
I agree with the others, the lettuce leaf trap (you can try other non-messy veggies too) is frequently reported here as working. Placing a tea saucer under the veggie can sometimes help to catch more snails as they start to fall off when you raise the thing out of the water the next morning. Be sure not to put the snails in the indoor trash or they will crawl out.
Perhaps even somewhat more important than the lettuce thing is your attitude. Instead of looking at your tank and wondering what the right "snail solution" will be, its better to think of snails as an ongoing part of tank maintenance, just like nitrates building up or something. First of all, examine your feeding habits (the members can help) and be sure you are not overfeeding. Feeding once a day only the amount that can be consumed in 2 to 3 minutes and having an occasional non-feeding day is enough. Examine your gravel-cleaning technique. The weekly water-change-gravel-clean should deep clean all the gravel and an attempt should be made to stir up and try to get out as much loose debris as possible. If 50% of the water is removed doing this then so much the better! Most important, don't be shy about crushing the snails against the glass and letting your fish have a treat eating them. It takes longer for the snails to grow than for you to crunch them, so if you do several every time you can, you will eventually win out.
OK, now lets move on to the other things that perhaps are a larger concern (since snails are perhaps nice looking but are not really a danger to anything!) You describe 4 potentially large fish in a "tiny tank" and I think its important that you tell us the numerical volume of the tank and the total fish and sizes! The members can help verify whether you are "overstocked." Likewise you may need to describe your filter and your water chemistry (as reported by results from liquid-reagent-based test kits) so that we can be sure your fish stock is not overloading your "biofilter." Its easy for this to happen in situations with a small tank and small filter!
Algae is encouraged by two things, ammonia and light. Ammonia is always present in a running tank. It passes from waste sources (fish respiration, fish waste, excess fish food and plant debris) to the filter where it is eaten by the ammonia oxidizing bacteria present in the filter. When this flow of ammonia is "under control" (from the fish standpoint) then our liquid-based ammonia test kits will register "zero ppm" ammonia. Sometimes to the surprise of beginners, this "zero ppm" doesn't really mean "zero" of course but just very low level and if the water circulation in the tank allows "still pockets" then there can be enough ammonia to feed algae in places in the tank (margins between substrate and glass are good examples) and if enough *light* is present (especially sunlight or all-day tank lighting!) then algae are encouraged!
So the algae is a sign of something! Its asking you: Too much light? Too much leftover food in the gravel? Too many big fish peeing in the water? Not enough good circulation of the water throughout the tank? Not a big enough biofilter?
In your web search to get here you've stumbled across a great hobbyist site! The TFF members are great at stopping in to chat and spending part of their online time helping other members with problems and the sharing of experience. I hope you get to the bottom of your problem and that in the process your experience with your hobby is enhanced!
~~waterdrop~~
