newb woes

Question
ok for many years now, i have been fish keeping but everytime, my fishes keep dying from disease or some other reasons...the bottom line is, they jus keep dieing even though my ammonia nitrite and nitrate lvls are all good..that is ammonia 0, nitrite 0 and nitrate 10-15 ppm..i have a 20 gall tank with a cycled HOB filter with a bio wheel...my tank is also planted...my few questions are :

1) how to prevent fish diseases?
2) how often to do water changes and by how many percent?
3) do i need to vaccum the gravel to suck up the fish poo or jus lift the water siphon tube in the water?
4) what to do when fish is sick
5) how to check if fishes are healthy at the store.

and finally...

6) what should i do when bringing new fish home?

pls help me as much as possible ..thanks

Answer
Hi Charistance
Sorry you've had problems.  I know it's frustrating, especially when you don't what's going on, and nothing's obvious to figure out....What kind of fish were you trying to keep?

1 & 5.  Best way to prevent diseases or prevent the spread into your tank, is to buy from reputable shops-which can be a challenge sometimes.  I've always heard or read that most stores have one big filtration system for their tanks.  This is pretty obvious to see when you look at the tanks and the filters they have.  So, if one tank has a disease like ich, it's very likely all the tanks will have the problem-even though you may not see it in all the tanks.  
So, look through all the tanks and check out all the fish to make sure they all look healthy.  Sure, one fish may have a case of ich, but if several tanks have sickly looking fish-don't buy from them.  Another thing to watch out for, are there lots of dead fish in the tanks?  If so, don't buy.  Now, it's normal for there to be dead fish in the groupings they buy, but if the fish are left in there and scooped out regularly, the live fish will feed on them-which is a great way for diseases and parasites to be spread amongst the fish.  Last chain pet store I went to, I actually saw several tanks with fish almost to the bare bones in them....which means, the dead fish had been in the tank for quite awhile.  

Another thing that's a very good idea, get a separate tank, I usually recommend a 10 gallon tank, and use it as a hospital/quarantine tank.  All you need is a bare tank-no gravel or decorations, and a basic filter.  A heater would be good as well.  QT all new fish for at least 2 weeks in that tank.  Watch them for signs of diseases/parasites.  What I also do now, is automatically treat those new fish with rounds of antibiotics and meds for parasites.  I use an organic ich/parasite med so it's not as harsh on the fish, and an antibacterial food for feedings.  Certain fish that are prone to internal parasites, I'll also feed an antiparasitic food as well.  Last few fish I bought I did that for was some loaches that are prone to internal parasites, and they ended up dying on me....and I'm so thankful I did QT them and not put them in the main tank with my other loaches.  The store said they ended up losing their whole batch as well.  

2 & 3.  For a normal stocked tank, I recommend changing out about 25% of the water once a week.  In doing that, you should be using a gravel vacuum/siphon to clean the gravel.  Obviously, that will also remove some water at the same time-which is how a water change should be done.  So, yes you also want to weekly clean the gravel and use the gravel vac to suck out the poop and uneaten food that gets trapped down in the gravel.

4.  When a fish is sick, it's usually best to quarantine the fish in a separate tank.  The exception to this, is if the fish has dropsy or popeye-which isn't contagious but more a sign of poor water quality, or for ich-because it's a problem in the tank so the whole tank needs treating regardless if the other fish are infected or not.  Then, you need to figure out what's causing the illness in the fish.  9 times out of 10 when a fish isn't acting right, it's a water quality issue.  So obviously fixing the water quality by doing a water change can help.  If the fish does have a bacterial infection, usually a round of antibiotics is needed.  For parasites, usually an antiparasite med is needed-just depends on the type of parasite you're dealing with.  I also don't recommend using meds until you're sure about what you're dealing with.  Fungus on a fish may be from a fungus, but it's usually going to be a bacterial infection-so fungal meds won't work and all you've done is stress out the fish with needless medications.  Scratching is usually a sign of parasites, but can also be from poor water quality.  A little investigation needs to be done before using medications, because some meds are just as deadly to the fish as the diseases.  

6.  New fish should as I mentioned, be quarantined-ideally.  You have the fish in their little baggie, you want to float it in the tank for a few minutes.  This allows the water in the bag to adjust to the tank water temperature.  Then, you want to slowly over the course of at least 30-45 minutes(longer for sensitive fish)add a little bit of tank water into the bag(you can also use a bucket for this-obviously dump the bag of water and fish into a bucket).  This allows the fish to adjust to any ph differences from the pet store water and your house water-which is most likely going to be slightly different.  When you're done letting the fish adjust to your tank water, net the fish out of the bag/bucket and add it to your tank.  Never dump the pet store water into your tank-that's how you spread parasites, bacteria, algae, even snails into your tank.

Hope that helps!  Let me know if you have more questions.

Christy