In any case, if you add a few fish and then let the tank develop enough helpful bacteria to handle them before adding more, things should go well. Adding fish in stages is a safe and easy way to cycle a tank, and chances are good that you won't lose a single fish during the process.
*sigh* this is gettin' way off base.. i have done research, and it is, despite popular opinion, possible to do a fish cycle and not have the fish die. i have already decided to do a fish cycle and i did not post for people to try to persuade me otherwise and/or make me feel bad about doing a fish cycle. i just wanted to know if 3 platies would be too much to cycle a 10 gal. tank. that's all. so can someone help me?
Folks are trying to help you. The first statement above is absolutely incorrect, to do a fishy cycle without harming the fish you will need to do a lot of work, and will create a lot of unnecessary risk. Your statement is correct that a fishy cycle can be done without harming fish, but for someone to say it's easy is wrong. In a 10g you probably won't want to start with more than 1 small platy, anything more will increase work and risk. Feed extremely lightly, and monitor water regularly. If you see ammonia or nitrites above 0.25 ppm do a water chang to get them back down. After the 4-6 weeks it takes to estabilish (probably closer to 4 with fishy) then you will want to add one fish at a time and allow 1-2 weeks for the bio-filter to grow to compensate. A 10g is small enough that it takes very little bio-load to cause spikes. If you were dealing with 30 plus gallons, 3 platies would be fine.
There is a far cry of difference between not harming fish and actually killing fish. Ammonia causes gill burn, which in turn causes scars which in turn permanently effect the fish's long term health. The effects of nitrite are not as permanent, but you will not have one without the other so when nitrite spikes, and gills are already burnt, the fish suffers more. Many fish will live through this, and many hobbyists tend to reccomend it, with the attitude that it's O.k. to bring an animal home, put it through hell and then replace it when it dies. If that is what you want to do then you are probably seeking advice on the wrong board. If you want to fishy cycle, then you have some work cut out for you, and even with experience and a lot of care you run some risk. Almost all of the old timers here have fishy cycled, many of us have done so without any spikes or damage to our livestock. We have also all found better safer easier ways because experience has taught us that it isn't easy to do a proper fishy cycle.
If you want to do fishy, and you want to do it without harming your fish proceede very slowly and test often. Remeber that excess food generally causes more problems than fish will in new set-ups. It is close to impossible to starve a fish, but it is also difficult to resist the urge to feed your new fish too often.
To put the fish per gallons in perspective, when I had 55g tanks, I cycled with two or three juvenile giant danios (about 2" long) I fed once every three days. If I ran more fish or more food I had to do frequent water changes during the early part of the cycling process. Then I would add no more than one or two fish per week until stocked, and not up my feeding until a week or so after the last addition. To do things right you need to proceede so slowly that you don't see spikes at a measurable level, and when you do that it is hard to recognize what is going on and usually very tempting to goahead and add another fish, which will in turn cause spikes, increase work, and potentially damage your fish.
There you have it in a nutshell.
Bio-spira is to my knowledge the only legitamite cycling aid out there, all others are either snake oil, or they cuase more trouble than they are worth. In addition it is quite possible to buy a bad bottle of Bio-spira, so be prepared to get another bottle if it doesn't work the first time. If bio-spira is good, it will give you the bacteria you need and pretty much instantly cycle your tank for a full bio-load. The risk involved lies with the bio-spira, and if it fails you will be doing multipl daily water changes to keep from killing the fish. It is better to put in the bio-spira as is done with fishless, dose with amminia and test the next day. If the bio-spira is good the ammonia will be gone, and you can safely put your fish in the tank. If the bio-spira isn't good, you will no via the testing and you won't have atank full of fish to worry about caring for.
Dave