If you are putting the filter in with new-born fry or a tank that is 5 gallons or less, then the 50ppi is the best choice, otherwise, the 15/30ppi filters are a better deal for a much larger size.
These filters are fully "cycled", containing all the many species of nitrifying bacteria a fully established aquarium needs. Once put into a tank, the tank becomes instantly cycled. From that point you only need to let the bacteria volume grow to handle the waste level of the tank. Nitrifying bacteria will double in population approximately every 24 hrs, Therefore, 3 days after introduction to a new tank, the bacteria can be 8 times the number if the waste level warrants it.
All of our tanks contain snails, so these filter may have small snails and/or their eggs on them. Also, they contain the decomposed waste generated by the plants and animals in our tanks. They will be "dirty". This is not harmful and is what happens in biological filters. The nitrifying bacteria are short-lived and create a large amount of bio-floc as it dies off. All of this is normal, but if you are offended by this, please do not order. Filters that are too clean, can be a reason if you are having a difficult time establishing a bio-filter, since cleaning a filter too much harms the bacteria.
There are likely hundreds of species of nitrifying bacteria in a tank that has been "established" for over a year. This is why those commercial "starter" products with a dozen or so species, don't work very well. Our filters contain all the species in balance and is why they work so much better.
The bacteria survives shipping quite well, but faster is better when deciding on the shipping method during checkout. If the filter freezes, the bacteria may be killed so pay attention to the weather, here and where you are. Try to avoid having the package sitting outside after delivery. It can be risky to order when night-time temps are much below 0 F. We are in a potentially very cold area of the country (zip 14760). Heat does not bother the bacteria. In transit, the bacteria goes into a dormant state and reactivates in the presence of ammonia and nitrites. A percentage of them will survive for quite a long time out of an aquarium, so even with delays and bad weather, they persist and will often still work, though a bit slower.
The filters vary in size and shape within the range listed. All of our Active filters are the "Internal Weight" filter version. We have too many filter types to have them all in an "active" version and this type is easy to ship without water leaking from the bag.
These are "established" in our disease-free aquariums. This assures that no pathogens are transferred to your fish. This is safer than using a filter out of your own tanks, since few, if any are disease-free. These filters have been run for about a month in our super healthy tanks to insure the nitrifying bacteria are well established within the sponge.