Take Wilder's advice on how to treat. (BTW, nice fish in your avatar)
Now because you say you are new to fishkeeping and have not tested your water, I think I should explain a few things so that you can understand what is wrong. But first, a few questions:
What size tank do you have?
What sort of filtration do you have?
Why did you leave the tank up and running since November? Was this to cycle? Did you add ammonia?
What are the 'white fish'? I am completely lost as to what these might be - can you specify whether they are tetras, danios, barbs, corydoras catfish, rasboras or something else? Is this what they were called at your LFS? Do you have the scientific name?
Also, what are the 'sucking fish'? I ask because there are certain species that are not good for your average aquarium - several kinds of plecos grow to over 18 inches long and CHINESE 'algae-eaters' grow to 10 inches and will kill fish and become very aggressive and territorial as they mature. It is very important to know exactly what you are buying before you do get it.
A note on your guppy/betta combo - this is generaly advised against. Male bettas will habitualy kill guppies which they often mistake for rival males due to the vivid coloration and finnage. Now that the betta has died, I would not replace him.
Now, as regards your initial questions (and what I was gonna lecture you on
), the likely cause of all these deaths is the fact that you have added so many fish at once.
Fish produce waste. Waste contains ammonia. Ammonia is highly toxic.
Luckily, there are good bacteria that convert the ammonia to nitrIte. Unfortunately, nitrIte is also toxic.
Luckily, there are other good bacteria that convert nitrIte to nitrAte. NitrAte is a lot less toxic. A single partial weekly water change of around 20-25% with CE-CHLORINATED water is all you should need to keep nitrAte down.
This process is called a 'cycle' and is essential to your fish' health. It takes about 6 weeks to be completed. During these 6 weeks, if you test your water, you would see the ammonia levels go up and then gradualy decrease. As the ammonia decreases (is converted to nitrIte), the nitrIte increases. Eventualy this nitrIte also starts to decrease and nitrAte starts to go up as the bacteria convert the dangerous nitrIte to nitrAte. You remove the nitrAtes with a partial weekly water change as mentioned before. A level of under 40 ppm of nitrAtes is considered safe. However, under 20 is best.
The reason you must use DE-CHLORINATED tap water is that chlorine, besides being irritating for fish, will also kill off the good bacteria - leaving the deadly ammonia and nitrIte toxins to kill or weaken your fish. Weakened fish are then more likely to fall ill.
What you need to understand is that the good bacteria will colonise your filter media and gravel and use ammonia for food, multiply until they are enough to handle that amount of ammonia and convert it to nitrIte as a waste product. Then other bacteria will also colonise and multiply until they can handle the amount of nitrIte and convert it to nitrAtes.
The ammonia the bacteria use, however, doesn't only come from fish waste but from rotting debris and food as well. This is why it is essential to not over-feed and to remove any dead fish mmediately. It also makes it very important to remove any debris or uneaten food from the substrate. Provided you don't over-feed, a gravel vacuum (gravel vacuums can be bought from most any LFS) every week (with your partial water change) will keep the gravel clean of rotting debris. You would be surprised as to how much waste and stuff accumulates in your substrate if you do not clean it regularly.
Occasionaly, you may also want to clean out your filter - NEVER wash it under the tap! Chlorine in any water that has not been de-chlorinated will simply kill off all the bacteria that have colonised your filter media and cause your tank to re-cycle (so more ammonia/nitrIte again). This can easily poison your fish. So to actualy clean out the filter media, remove it to a bucket of old tank water (from a water change) and carefuly and gently rinse it out. This should be enough to remove most solid waste but not kill off your bacteria. If you ever find you must replace the old filter media with new, NEVER replace all of it at once. Always take part out at a time. Taking all of it out would remove all the bacteria and, again, be dangerous for your fish and water quality.
Now that brings me to adding lots of fish at once - because you have suddenly added fish, you also have more waste. That means more ammonia is produced. That, in turn, means you need more bacteria to keep up with the ammonia and the nitrIte and convert it all to harmless (for our purposes) nitrAte. Unfortunately, the good bacteria don't grow too fast so they need a chance to catch up. Unfortunately again, the fish will suffer if the ammonia and nitrItes do not decrease immediately.
Basicaly, your tank is now not cycled. In a cycled tank, ammonia and nitrIte readings are a steady 0. The nitrAtes remain detectable but can vary greatly depending on maintainance and the like. Heavier stocking (more fish), over-feeding and lack of gravel vacuuming/water changes will cause an increase in nitrAtes while plants (especialy fast-growing), under-feeding, under-stocking and regular maintainance will keep them under control. Plants will use nitrAtes up. So will algae - which is why algae grows best in tanks that are over-stocked or over-fed. Water changes, however, are the only real way to keep them low.
I can't emphasize the importance of regular water changes enough. Clean water is the key to healthy fish. In an un-cycled tank (which contains ammonia and nitrIte) you should do even more water changes. Even the slightest level of ammonia is harmful to fish. In a cycled tank it would be converted immediatly to nitrIte and immediatly to nitrAte so it wouldn't cause problems. Small amounts of nitrIte are also deadly but not to the same extent. NitrAte is almost harmless (note the ALMOST) at lower levels.
I think that pretty much sums it up... You need to do a large water change to lower any dangerous levels and then go out and buy yourself a liquid-based test kit for ammonia, nitrIte and nitrAte. pH is also useful to know (though that doesn't have anything to do with the cycling process). Also get a gravel vacuum and de-chlorinator (and some buckets - you'll need those
) if you haven't got some already.
One last thing - stocking levels. I don't know exactly what you have in your tank or what size your tank is or your filtration system or that matter BUT I can give you a little guideline to help you out. '1 inch of ADULT fish per gallon'. The ADULT means that you must know the ADULT size of your fish and calculate according to this so that for every inch of adult fish, you will have at least a single gallon. There are other things to take into consideration (like ornaments displacing water, filtration, maintainance etc) but this guideline is pretty accurate for fish that grow up to about 4 inches and with weekly 25% water changes and normal maintainance routines.
Now that I'm done lecturing
- excuse me if I've told you anything you were already aware of - I suggest you read through the first 3 links in my signature to familiarise yourself with cycling, new tank syndrome and what you should have done before adding fish - fishless cycling (if you didn't that is).
Something I've remembered - the good bacteria need oxygen (that's why they grow so well in the filter media where water is constantly circulating and passing over them) and ammonia to survive so if you leave a cycled tank without fish for more than about a day, the bacteria will have died off significantly and adding as many fish as you had before is likely to cause an ammonia and nitrIte spike (a 'spike' is what we call it when the levels rise as the toxin acumulates) and cause problems for the fish.
Also, your filter media will smell like wet soil once your tank matures - don't mistake this as meaning it is dirty. It is rarely necessary to replace filter media unless it begins to biodegrade (in which case replace part at a time as described earlier).
What else is there?
Ah yes, you can speed up the cycling process by 'borrowing' old filter media or gravel from an established tank (either at your LFS or from a fish-keeping friend). This will introduce some of the beneficial bacteria. Ornaments - rocks, plants, little castles and 'no fishing' signs
also serve as a surface for bacteria to colonise so using these from an established tank would also help somewhat. However, filter media and gravel have a larger surface area for bacteria colonies so are generaly more effective. Make sure the tank you are getting these from are healthy. You can return the stuff back once your tank has fully cycled. Gravel that doesn't match your own can be hung in a new stocking inside your tank untilt the bacteria have taken hold (your tank's cycled). Adding old water from an established tank will NOT help and may introduce disease so don't bother.
