Each system is two fold, they are designed to catch all the waste that fish produce and other dead plant life, leaves and sludge. It is deposited in  various medias depending on what type of filter you buy. They also build good bacteria's creating a biological filter to keep your pond running balanced. Once the waste products are caught in the filter they begin to rot and build bacteria's. These good bacteria's are essential  to a balanced system. However you do need to clean off the excess of detrius that builds within the filter media. It is in your best interest to clean off the outer layers of sludge that build and leave the inner portions to be deteriorated by the bacteria's that are building. Never wash out filter media with tap water as you will also be washing out all the good bacteria within them. Use pond water to rinse the filters as necessary. This way you will cycle your system a lot faster. If your filters begin to smell foul like rotten eggs or sulfur then that is not a good sign and means that you have not cleaned them enough. It can be a tricky balance. 

When looking for a filter system keep in mind that you want it not only efficient but easy to clean. You will have to search around a bit to find out what will work best in your particular system. Make sure that it includes a backwash feature and a drain off for sludge. 

When you bring home a new fish it is not used to your water. You must acclimate the fish to your existing conditions to make a smooth transition. When you bring the fish home float the bag for at least 20 minutes to regulate temperature then slowly add some of your own water into the bag. Repeat adding your water into the bag for about 4-5 times approximately 10 minutes apart. If your bag gets to full dispose of some of the water but not into your pond or quarantine system. Once you have adaquately adapted the fish to its new water, pick the fish up with your hands or use a sock net and place it in your system. Dispose of all the water in the bag.

Under ideal conditions you should ask what type of environment the fish had been living in when you buy it. In other words, is it city water, what is the temperature, what is the PH level etc. You probably won't be able to re-create the living conditions that the fish has been in so you must get your fish used to its new environment but do it slowly. The fish should remain in quarantine for at least 2 weeks to rest before adding it to your main collection. This way you will not risk infecting your existing fish and the new fish will have time to adjust to its new living conditions. If at all possible buy at least two fish to keep each other company in a quarantine system. Koi do not like to live alone and will sulk about it. It can not be stressed enough to put a net over the entire quarantine system to avoid the fish jumping out when it first arrives. For that matter, a net should be over your main system for these same reasons and also to avoid predators from coming in for a visit. Fish are looking to go home and will try all angles to do so. They are under a tremendous amount of stress with the move. 

The first stressor is being netted. Fish just are not used to being out of water. That can effect the total system of a fish and send its immune system into shock therefore being susceptible to many illnesses. When transporting a fish it should be caught in a sock net to minimize damage and trauma. If at all possible pick up the fish and move it rather than net it. Also the new water is carrying bacteria's that your fish may be used to but the new fish has never been exposed to. You need to destress the new fish and make it as comfortable as possible so that its immunity has enough strength to ward off any new bacteria's that it may come in contact with in its new environment. Once you see over a period of 2 weeks that your fish has not succumbed to any illness you can safely add it to your main system. 

Do be vigilant about using preventive measures as in the case of using Koi Zyme for combating aeromonas. Do keep your water clean with good filtration. Do test your water regularly for ammonia and nitrites. Do inspect your fish everyday to be sure there are no problems brewing. The behavior of your Koi will let you know that something is wrong. Act quickly if a problem arises. Do add strong aeration to your system by way of air stones or waterfalls. Do add plants to your pond for a better balanced system but beware that Koi LOVE to eat them. Large smooth pots are the best set up on a shelf to minimize the damage to them by the Koi. Do have a bottom drain installed in your pond. 
 

Netting is dangerous, harmful and the most un-natural thing for a fish to endure. It also takes a toll on one of there best defenses, that being there slime coat. We do of course need to catch them some how but there are proper ways to do so. The technique is to come up under the fish slowly and lift them into a bowl that is floating in the water with them. From there they can either be lifted out to a transport bag with your hands or by using a sock net. The sock net is much less harmful and injurious to them than lifting up out of the water in a regular net. Fins don't get caught and they literally slide out into there next destination. If you are not bagging them for transport than they can be carried in bowl or bucket to another water source. 

Putting a fish in water that it is not use to also causes a great deal of stress. They are used to the water chemistry of where they were living. Every time a fish is moved to another water source the chemistry can be drastically different. It takes them time to adjust and build up immunities to the bacteria's in that new water, rebuild any damage done to there slime coat and to feel at home.

The chemistry of fish is that of flight or fight. In the wild they have to be aware of predators, they are aware of sounds around them that warn of danger. Fish do startle and when that happens they release adrenaline just as we do as humans but the only difference is that we can settle our chemistry down in a short period of time. A fish can not do that. It takes a fish many hours to reduce the adrenaline in their bodies and while in this state they can go into shock. This can kill a fish. Cracking ice on a frozen pond above them, chasing them with nets, being over crowded, taking them out of their water to medicate them, and even transporting them all cause a chemical imbalance. All this creates a stress factor.

When fish spawn or breed, that is also a great stressor. The male chases the female so much that he can actually exhaust her to death. That is why brood stock should be in excellent condition to endure the breeding process. Time needs to be spent afterwards to de-stress the fish or they will become ill.

A fish's best protection is there slime coat, a good diet and healthy water. If their slime coat is down and there is bad bacteria in the water, or if they have an injury and stressed it all sets them up for a major problem. They will get sick. Nature has its ways of protecting and preparing the fish for all kinds of conditions and it is us humans that are usually the culprit in making them sick. We should be more concentrated on keeping there conditions, water and environment healthy than on the fish themselves. When we are the keepers of the water our fish will thrive.