Everyone's experience of fishkeeping is different from the next person, you can't really put a figure on "normal loses" because some people simply experience more bad luck than others in the hobby.
If someone lost 20 out of their 30fish in just a couple of weeks, an outsider would probably look at that person with disdain and shock, however its not until you know the full story that you can judge how responsable the person was for those fish deaths- they may have encountered a particularly vicious strain of whitespot or columnaris which wiped out almost all of their fish in just a week and the person did all they could to save the fish but to no avail, or it could have been the opposite, where the fish died because of the persons negligence over their care etc.
I have about 170 fish, more or less (although its difficult to keep track of exact number them with all the fry being born and growing up all the time), i used to encounter problems and deaths with my fish a lot of the time. The problem i had was new diseases and parasites being introduced to the tank all the tank via new arrivals and having no where to isolate sick fish.
After a while, i set up a 5gallon quarentine/hospital tank- since having this tank (which has been set up since around the beginning of this year and at various points before hand), i have dramatically reduced the ammount of deaths and problems encountered with my fish.
The main fish i used to often have problems with (actually they were the only ones i ever had problems with really) were the guppys- i keep a great deal of guppys, it is well known that these fish are not as tough as they once were in the early days of the hobby and due to the often very bad conditions they are raised in at fish farms, they tend to be genetically weak (from inbreeding) and riddled with diseases and parasite. I put a lot of time into breeding my guppys, and to help prevent them from becomming inbred, every 2-3months i will buy some new females or males to help keep the gene pools of my guppys fresh and strong. The problem with buying guppys on such a regular basis though was i was always introducing diseases and parasites to my current guppys, even if i quarentined the guppys, the new ones just seemed to be very weak and prone to diseases and parasites for the first month or two (after that, after they had been cared for well and fed a good diet for a while, i rarely encountered problems with the new editions).
How i solved the the problem in the end was to simply use my quarentine/hospital tank (which is very well filtered and planted) better by quarentining new fish for up to 3weeks (sometimes even longer than that) and to move any fish whose health looked dodgey (even if the fish was just showing the slightest meager symptoms) to the quarentine tank imediately, and to only put the fish back in the main tanks after at least a week after it has got better in the quarentine/hospital tank.
I also feed my fish more frozen foods (better than dried) and regulate their diet better, some days the fish will get 3meals a day, but usually 2meals, but also quite often they will just get the 1meal- by making the fishes diet carefully more irregular and varied, it ensures that the fish do not become constipated and their diet reflects the foods they'd get in the wild and the quantities they'd get them in etc.
In the last 2-3months, i have only lost 2 old male guppys (their deaths were related to their old age) and 1 danio which didn't make it past the quarentien period. Out of 170+ fish i don't think this is bad at all
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So basically using my quarentine/hospital tank a lot more efficiently and often and for much longer periods of time and regulating the fishes diets better i have cut down on fish health problems dramatically in my tanks.
I know running an extra proper tank set up with lighting, heating, planting and filtration can be a pain in the bum for a lot of people to bother with and to set up such a tank will probably cost you a fair amount of money even if it is just a small tank, but the value in having a quarentine/hospital tank is priceless in my opinion, especially if you have fish like guppys, platys, mollys, oto's etc (basically fish which have a reputation for carrying diseases/parasites a lot or for simply being weak/fragile fish which die easily etc)
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The size of the quarentine tank you get should reflect the size of the sorts of new fish you want to buy. A 5gallon tank is fine for small fish 3inches or smaller, but if you are going to be buying 7inch long cichlids or whatever a 20gallon quarentine/hospital tank is probably much better etc.
Another thing i would definately advise people to do if they can is to stock up on different types of fish medicines, because the sooner you treat diseases/parasites in your tank the better the chance you have of treating your fish successfully- some diseases/parasites symptoms can come about very quickly in fish and can kill the fish in just days, so every day counts when it comes to treating fish sooner. It is always handy to have meds which treat external parasites (like flukes and velvet), internal and external bacterial problems, internal parasites, and an anti fungal med etc.
Even if your fish is only showing one odd symptom which does not point to anything specific, its important that you separate the fish from the others ASAP none the less just to be on the safe side- who knows, you could end up saving your tank from something like a particularly vicious strain of columnaris or something like that. With some fish diseases/parasites it is better to treat the entire main tank as a whole (like in the case of a whitespot outbreak), but for many diseases/parasites it is better to isolate the sick fish and treat it that way (like in the case of body fungus or septicemia).
So basically, i cannot express more how important it is to keep a very close eye on your fishes health- the more fish you have, the easier it is for sick fish to go unnoticed and for diseases/parasites to spread quickly. Understocking your tank will reduce the chances of diseases/parasites spreading very quickly from fish to fish, while doing larger and more water changes on a more regular basis and keeping the substrate cleaner more often in the tank will help reduce the background levels of any diseases/parasites in the tank. A sand substrate is easier to keep clean and is less likely to harbor columnaris (because the flexibacteria columnaris often starts its life in decaying matter like old fish food- gravel substrates trap a lot more waste in them than sand substrates).
Over-filtering the tank is also beneficial for many fish, there are a few fish which don't like strong currents in the water in their tanks (for example like gourami's and betta's), but for many fish they do enjoy a strong current in the tank- having a more oxygenated tank will help reduce the stress levels of the fish and make situations like medicating tanks easier (since many meds reduce the oxygen content in water) etc
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Also pay good attention to the type of habitat you provide your fish too, planting is very beneficial for many types of fish and so is stuff like caves made out of piles of rocks/driftwood etc.
When you pay more attention to these many aspects of fish keeping, it can be amazing the amount you can reduce the amount of health problems and deaths encountered with your fish etc
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