Buying Filtration
Which one to buy? Which type?
The best web site for a comparison and education on almost ALL filtration types is KoiFiltration.com
Filtration is an easy area to get screwed. There are a LOT of unscrupulous dealers out there.
An amazing number, whom you might call for a part or after sales followup - and they say "We don't provide support, we're an internet only company, you will need to call the manufacturer."
Filtration is probably the first and most common downfall for the Koi, goldfish and pond fish hobbyist. The primary reason is that Koi, goldfish and pond fish are fun to feed, they can eat a prodigious amount of food if allowed to do so, and the concomitant wastes, which the Koi, goldfish and pond fish produce, present a challenge to any traditional filtration scheme.
Filtration occurs on two levels. The first level is the mechanical phase in which visible impurities and particles are removed by a mechanical baffle or screen. The second level of filtration is the actual chemical reduction of waste in the water. In this discussion, we are primarily concerned with the elimination or reduction of nitrogen from the water. The nitrogenous waste products come from the proteins that the fish ingest, and is liberated into the environment in the form of Ammonia, which is reduced by bacteria living on the surfaces of the filter media. We will discuss this again, in more detail, in the section on “The Cycle”
Bead Filtration is relatively new. The pioneers of this technology simply forced large amounts of water through a “cake” of floating beads. The bead-pack would remove the particulate debris and once the nitrifying bacteria have grown up on the surface of the beads, the chemical wastes are reduced as well. The very nicest aspect of the bead filtration systems is that the hobbyist can “backwash” the water flow through the beads, eliminating the retained debris as easily as turning a valve.
There are several manufacturers who are
doing a good job at this printing, including Aquadyne, FluidArt, Challenger, and Hydrabead.
CALL YOUR DEALER before you buy on the internet. If they are unresponsive or cannot offer support BEFORE the sale, imagine AFTER you've spent a thousand dollars!
You could research the specifications of each system using the Internet.
The efficacy of the bead filters is unparalleled. They are expensive to obtain. It is not unreasonable to expect to pay hundreds for the system. You should remind yourself that there are no wear-parts and no expendable resources in bead filtration. Once you own the system, it runs indefinitely without real work.
The advantages of Bead filtration include ease of operation, long life spans, unparalleled carrying capacity, and crystal clear water. The disadvantage of the bead technology lies only in their price tags.
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PondCrisis.com
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KoiCrisis.com
Koi Crisis has a symptoms chart by system you can choose the symptom by fish part, and resolve a lot of Koi pond fish problems or at least, learn about them understand how to remedy them.

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What does "Domestic" koi mean? Why would you buy that kind? How do you pick good and healthy ones? Who sells them and where do you find the best ones?

Buying Imported Koi
A Japanese or Israeli imported Koi is a beautiful thing. Why would you buy one of those? How do you identify a "good one"? And what kinds are there? Who would you buy one from?
