I'm assuming the tank dimensions are in inches (11 inches wide x 30 inches long x 20 inches high)?
If you don't have a gravel cleaner, I would suggest you get one so you can clean the substrate and do a water change at the same time. That way you don't have to do a complete water change and clean the tank once every few months. Instead you do a 50-75% water change and gravel clean once a week and that helps keep the tank clean.
The following link has a picture of a basic model gravel cleaner. It's located about 1/3 of the way down the page. If you can't get a gravel cleaner, you can make one from a plastic drink bottle and garden hose. See below for directions.
Home made gravel cleaner
Get a 1, 1.5 or 2 litre plastic drink bottle (like a Coca Cola bottle).
Remove the cap and plastic ring from the top and throw these 2 bits in the recycling.
Cut the bottom off the bottle and throw the bottom bit in the recycling.
Get a length of garden hose (or clear plastic hose) and stick one end in the top of the bottle. The other end of the hose can go into a bucket or if you use the whole garden hose, run it out the door onto the lawn.
And that's it, one home made gravel cleaner.
Put the plastic bottle in the aquarium and fill it with water.
Have the end of the hose in a bucket or outside.
Start the syphon by sucking on the end of the hose or check YouTube for how to use a gravel cleaner.
When the water is draining out, push the bottle into the substrate (bottom of bottle goes into substrate) and lift it up. The gravel will circulate inside the bottle and drop down, while the gunk gets drained out with some water.
Repeat the process and clean different areas of the gravel.
When you have drained half the tank, stop gravel cleaning and refill the aquarium with dechlorinated water.
If you don't have any buckets specifically for the fish tank, go buy a couple of 10-20 litre buckets and use a permanent marker to write "FISH ONLY" on them. Use those buckets specifically for the fish tank and nothing else.
When you do water changes, you fill up a couple of buckets with tap water, add some dechlorinator, and aerate the mixture for at least 5 (preferably 30) minutes before you use that water in the aquarium. This gives the dechlorinator time to neutralise any chlorine/ chloramine in the water and make it safe for the fish. You can make up the water and let it aerate while you do the gravel cleaning. You can contact your water supply company to find out if you have chlorine or chloramine in the water. You normally use a higher dose of dechlorinator for chloramine.
The fish in the tank are a Silver shark, sometimes called a Bala shark. That is the big silver fish with black edging to the fins. They do best in groups of 6 or more and grow to about 10-12 inches long. It is going to get too big for your tank.
The small orange, yellow, black & blue fish looks like a male guppy. It is probably full grown.
You have algae problems (green stuff on the ornaments and glass) because you don't have many live aquatic plants in the aquarium. Algae is a simple type of plant that will grow anywhere there is light and water. If there are aquatic plants to use the light and nutrients in the water, then algae won't do as well. However, if there are no aquatic plants, then algae will grow.
To control the algae, either add some more live aquatic plants or reduce the light. Water Sprite (Ceratopteris thalictroides/ cornuta) is a nice floating plant that usually does well in aquariums. It will spread across the surface and reduce the algal growth. If you get too much, you can plant it in the substrate where it turns into a lovely light green shrub shaped plant.
Other good plants to try, include Ambulia and Hygrophila polysperma. They get planted in the substrate and usually do well.
Established filters should be cleaned at least once a month. Wash the filter media/ materials in a bucket of tank water and re-use the media. Tip the bucket of dirty water on the lawn.
Seachem Stability is a liquid filter bacteria supplement. It is used to help speed up the filter cycling process. I normally recommend adding a double dose every day for a week, then pour the remaining contents into the tank. Try to add the supplement near the filter intake so it gets drawn into the filter where we want it. Because your tank has been set up for a while, you probably don't need to use this stuff. If you want to use up the remaining contents of the bottle, do a water change and gravel clean, clean the filter, then the following day add the rest of the bottle to the tank. Otherwise just keep it in the fridge until you have an ammonia problem or want to set up another tank.
Seachem Amguard is used to bind ammonia and convert it into a less harmful substance. You don't need this unless you have an ammonia reading in the water. If you do get an ammonia reading, the best way to fix it is to do a 75% water change and gravel clean the substrate every day until the levels are back to 0ppm. Reducing feeding can also help. But if you have an established biological filter, you should never see an ammonia reading. You can probably put this in the fridge too.
Seachem Neutral regulator is used to stabilise the pH at 7.0, binds to free ammonia (same as the Seachem Amguard), and dechlorinates tap water, neutralising chlorine/ chloramine. Adding chemicals that adjust the pH and do all these other things is not really the best.
API pH up is usually sodium bicarbonate and is used to raise the pH. Unless you are keeping fish that live in neutral (pH 7.0) or alkaline water (pH above 7.0), and your water supply has a really low pH (below 6.0), there is no need to add this. It could also react with the Seachem Neutral Regulator.
Seachem Pristine is another type of bacteria that helps break down fish food and fish waste. It isn't needed and the bacteria develop on their own.
API Stress Coat neutralises chlorine/ chloramine and helps encourage a fish's mucous coating. It also appears to bind with heavy metals and cause them to drop out of suspension so they don't affect the fish as badly.
Most of this stuff you don't need. Unless you have heavy metals and lots of toxic stuff in the tap water, all you really need is a dechlorinator to neutralise chlorine or chloramine. If you have chloramine in the water, then you also need the dechlorinator to bind with ammonia and stop it from affecting the fish. The API Stress Coat and Seachem neutral Regulator both do this.
You should contact your water supply company (via telephone or website) and get a list of what is in your water supply. Post it here so we can check it for contaminants and other harmful substances. But you could probably just use the API Stress Coat and not need any of the other stuff.
These types of additives should be kept in a cool dry dark place so the ingredients don't break down. I used to keep mine in a plastic icecream bucket on the bottom shelf in the fridge. Just make sure children and animals can't get to them because some contain subtances that can poison you if you ingest it or get it in your eyes.