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The white things are probably planaria. They are harmless to your fish and are actually quite a treat for them. However, they are usually an indicator of poor water quality. A few good cleanings of your gravel should probably make them disappear. Try to only vacuum half of your tank per week though so you don't the balance of bacteria in the tank.
The green stuf is probably algae and won't hurt a thing. Most folks simply scrape it off because it is ugly. It is very normal for your tank to have algae. If you get a lot of it, it may indicate that you have too many nutrients in your tank, and you may need to bump up your water changes to around 50% per week. You also might want to take a look at your feeding routine, and determine if you are overfeeding your fish.
Don't change the filter media until it starts to fall apart or water will no longer run through it! If you are changing it weekly, I'm wondering if your tank is cycled. Simply get a special bucket used only for your tanks and put some old tank water in it. Then just take your filter media and/or sponges and rinse them in the old tank water. A lot of the brown crud will come off, but the bacteria will stay intact. Carbon is not necessary unless you are trying to remove medications from your water, or you are trying to get rid of a bad odor in the tank. I have had some of my filter media in my filters for over 6 months, with only occasionally rinsing them. Can you please post your
ammonia,
nitrite, and
nitrate levels of the tank?
Don't try and eliminate them with chemicals! Snails can be a problem, and also an indicator of overfeeding. You can eliminate them by picking a lot of them out by hand, and you can also put pieces of lettuce in the tank at night, and then simply remove it in the morning when it's covered with snails. You will never get rid of all of them this way, but you can somewhat control them. Another option would be to get some loaches for your tank. I would only get the loaches though once your tank is balanced, and if it's large enough. How big is your tank?
I had a serious snail problem in my 20 gallon long tank. I got 4 yo yo's (it's really better to have a larger tank for this species), and within a week, I no longer had a problem. There are many loach species to choose from if your tank can handle more fish. Just make sure you research the species that you are interested in thoroughly and make sure they are compatible with your present occupants (some can be quite aggressive), and that they won't get too big for your tank.
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