Welcome to FishLore! We will be glad to help you get this tank back in shape!
1. You are correct in that clowns do better in groups, and the tank is really too small for clown loaches. You can get by with having them in a smaller tank for a while because they are slow growers, but just for future reference, you really shouldn't buy fish that will outgrow a tank. Fish choices should be based on the adult size of the fish. I know you inherited this fish, but you may decide to change some of the inhabitants in the tank, so keep this in mind. Try and trade in the clown loach for credit at the fish store, or see if you have any fellow hobbyists in your area who might want to buy or trade it from you.
2. Gouramis are notorious bullies. My experience has been with them that the dominant one will relentlessly chase the others until they die from stress. You may want to try to trade in the 2 being chased, and keep the one if you wish. If you trade in the dominant one, one of the other 2 will just take over that behavior.
3. I know on the algae eaters you speak of, there are 2 species that are similar in appearance, but one species is much more aggressive toward other fish, and they are not very good algae eaters, especially as they get older. You may want to consider trading them in also and getting a bristlenose pleco instead for algae control.
4. Filtration of some sort is definately vital to the survival of your tank. If nothing has changed since you inherited it, my guess would be that the filtration is an undergravel filter powered by an air pump. It is pulling all of the waste down under the gravel. These filters were popular in the 70's, but now are not recommended by fellow hobbyists unless you use it in conjunction with some other type of filter, and reverse the flow so that the water comes up through the gravel and trash is pushed up to the other filter.
5. It sounds like the tank was not maintained like we recommend, so at this time, I would like to recommend that you get ammonia, nitrite, nitrate, and ph tests. The aquarium pharmaceuticals master test kit is the best bang for your buck in the U.S., but might be difficult or expensive in China to get. If you want to change stuff out, you can definately do that. That would also be a good time to either remove the undergravel filter (if you indeed have one), or reverse the flow and add another filter. With or without an undergravel filter, sponge filters are cheap and easy to use. First, take back or find good homes for the fish you no longer want in the tank. Make sure the ph of your tank water doesn't differ more than .2 on the ph reading. Then the day you plan on changing out the gravel, remove all the fish to a bucket or container that has water that is the same temperature as your tank. You may have to add some of the tank water if there is a substantial ph difference or you could shock your fish. You will also need to have a filter of some sort running in the container with your fish, or at least an air stone, but this is only if your fish will be temporarily housed for a couple of hours this way. Get a large enough bucket so any decorations you wish to keep can be placed in there also to save the live bacteria on them. Make sure your water in the bucket has been been treated for chlorine and/or chloramines before adding the fish or decorations. I would also grab up a good handful of the gravel and put it in an old nylon stocking and place that in the bucket also. This will be extra bacteria which will help you seed your tank once you change stuff out. Do what you plan on doing with the tank as quickly as you can, and don't use any chemicals if possible when doing so. If you use bleach, you will have to rinse, rinse, rinse, to make sure you don't leave any residue in your tank. This could be deadly to your fish. Once you've got it done the way you want it, fill it back up with water (always treat the water you add to your tank), heat it to the right temperature, and then slowly add your filters, decorations, gravel bag, and your fish. Keep close tabs on the ammonia and nitrites for the next few days. It will take time to get your bacteria level back up to be able to keep the ammonia and nitrites under control, so if the numbers go above 1 on either, you need to do a water change ASAP, or your fish will start getting sick and could die. If you need more information about the nitrogen cycle, just click on the link in my signature titled, "FishLore Articles for Beginners". It will help you a lot. Once you get everything done, post back with pictures if possible, and we will help you with maintaining a healthy tank. Later on when everything is settled, I would recommend weekly water changes instead of monthly.