KCGirl- Excellent points all. Allow me to respond. While I will cheerfully admit that crabs do indeed not breather their substrate, they do spend the most delicate parts of their lives there, namely, while molting. During that time period, the wastes and bacteria in the tank fundamentally affect the outcome of their molt, thus, it is just as important that the environment be safe as it would be for fish. I am also not looking at merely substrate when I say "the tank", I mean the entire environment. Thus, since the composition of substrate is one of the MAJOR factors in molt health, as that is when bacterial infections, death by toxin, and so forth occur, I do feel that it is just as important that they have a healthy substrate.
Water changes are indeed to remove bacterial concentrations and waste. Waste in an organic sense means the excretions produced by an organism, and that is what the nitrates are. The nitrates are a toxin in that they are a substance that is substantially harmful to the other organisms in the system. The toxins are not inert. Nitrates are toxins. The ammonia may be inert, at least most of it, depending on your tank's chemistry, but nitrates also kill. Also, the good bacteria live all over the tank, not merely in the filter, or even the undergravel filter media. These provide places for certain species of bacteria to congregate, but it is by no means the only place in which they live. And removing harmful bacteria is one of the points of the water change, as it is the bad bacteria's waste products that cause much worse problems than mere nitrates. If all that you are testing for is nitrates and ammonia, you are doing your fish a disservice. There are a great many compounds present in a fish tank, and many of them can be harmful.
I do believe in substrate filters. I call them plants. Just as mangroves and macro-algae provide natural means of nutrient export, removing things such as nitrates and the dissolved nutrients from food breakdown, plants in a crabitat serve the same function. Plants are the ultimate biological filter. I hold that crabs DO need a filtered substrate, as it will substantially help with the bacterial populations that could be harmful during molts, by removing the nutrients from the soil that they would need to flourish.
I agree that one can do massive water changes. The problem is, as you say, the shock to the fish. I never said that the shock came from having clean water, in fact, if you read my post, you would see that I said that it was NOT the new, clean, fresh water, but rather the sudden shock due to pH, temperature, and water hardness, just as taking crabs out of a nice cushy crabitat, putting them in a temporary holding facility, and then putting them back in a tank that now smells completely different and is all rearranged proves a shock. I completely agree that it is the biological filter crash and the dissimilar water parameters that causes the problems, as I said in my original post. I also agree that it is the change in conditions which causes crabs to have stress issues during deep cleans. Both of those were points that I made in my original post, and thus, I ask why we should subject them to those stresses, when it isn't necessary to remove them from the tank at all.
On your third point, I will MOST strenously disagree. Nothing can live in an actually sterile environment. We require bacteria in our guts to digest food, bacteria on our skin to handle environmental toxins, bacteria regulate just about everything for most forms of life on this planet. Allow me to suggest that with adequate biological filtration, 100% changes would not be necessary. Look at the example of the mangrove refugium, in which mangroves, macro-algae, and detrivores are used AS a filter for a main tank. The water in the refugium remains clean, with minimal changes required. I am not saying that the people who keep betta in unfiltered smaller than optimal conditons are bad people, but, personally, all my betta live in minimum 2.5 gallon planted filtered aquaria, and I would never put so much as a shrimp into an unfiltered tank. I use sponge filters for minumum water movement, since I do agree that filtration METHOD needs to depend on species, but I do not feel that need for filtration changes. Simply because a condition is adequate does not make it optimal.
I again, dispute that a crabitat is an unfiltered or closed environment. First off, there is no such thing as a closed system. Even when we were doing epitaxy in the vacuum trail behind the space shuttles, there was still contamination, much less in a tank in a house. That is the problem. Since we CANNOT maintain a sterile environment, all we do is create a bacterial vacuum, which nature WILL fill. I am not talking about a closed system crab only minimal bacterial system, I am talking about a well balanced vivarium, which is self sustaining. I am talking about large PLANTED, FILTERED water systems, a large PLANTED( i.e. biologically filtered) substrate area, and a background that allows more space for climbing and hiding, to mimic more closely the fact that most hermit crab species are fairly arboreal. An undergravel filter under the substrate, with filter media, and either a filter if you want one or bare minumum drainage capabilities if you don't, so that when the water from the plants drains, it doesn't cause root rot, and prevent the formation of anaerobic bacteria. There are a great many great sites on vivaria that explain the concepts and balancing issues in them.
As far as digging up a new crab, I immediately put ALL new crabs into ISO for at least a month, following fairly closesly the guidelines on coenobita.org. PPS has to do, in crabs, with sudden changes that they cannot adequately process, be it from change to a worse OR BETTER environment. Please check out the article at coenobita.org for more info. It takes them a while to adjust their metabolic functions. With Indos, for example, if they are suddenly dehydrated for a period of a few hours, lung and gill damage sets in. If they are rehydrated very slowly, they can recover, but if you rehydrate them quickly, they die.
As far as seeding the tank goes, again, with the plants and smaller life forms, we have enough material to feed the good bacteria. If you are concerned about obtaining pure strains of individual types of bacteria, and disease free insects, please allow me to suggest a scientific supply catalog, where you can be sure that they are pure sources.
With a well balanced vivarium, all you need to do is mimic the effects of wind rain and other animals by doing frequent small changes and cleaning. Vivarium keeping has been sucessfully done for decades with all sorts of animals, from some of the most delicate of dart frogs, to exotic lizards, to tarantulas. There are many more delicate species that simply thrive in such setups. In my mind, keeping a crab in a tank without live plants and live soil is no better than keeping corals in a tank without anything else, or keeping a betta in an unfiltered 1 gallon container. In all these cases, while we are trying to provide an adequate home for our pets, we are not doing the best by them that we possibly can. Crabs are FORAGERS. Without sufficient things to forage in and around, they lose interest in eating, leading to metabolic issues, as well as difficulties in molting for larger crabs. Please check out my blog, as well as some vivarium sites, for more info
I love this topic, as I find it great when everyone throws ideas out there. I think it advances our knowledge. In my opinion, friendly debate is ALWAYS a good thing