I have a 20 gal. tank that has been up since March and it seems like it keeps cycling over and over.
That's not a very long time--it's probably still cycling and not recycling.
Can that kill the good bacteria and make it cycle or is it from me vac'ing the gravel everyday to keep the tank clean?
Heat is fine, but stop vacuuming the gravel. You are removing waste that will give off ammonia to feed the bacteria which will eventually cycle the tank.
OK, I have OCD when it comes to my fish tank but I have 30-2 week old fry in there also.
That's not helping. If the fry survive (and they will since you keep changing water, but the tank will never cycle) count yourself lucky. Keep in mind, you may have many, many fry following since you are keeping livebearers, which store sperm and don’t need a male present. After awhile, you would have to add a male for more fry. Think about that later—give it many months.
The past week I noticed the ammonia rise and do PWC daily (along with gravel vac) and today when I tested, my nitirites are going up to between 0 and .25. I feed twice daily but the fry 3x's, the ph is 7.2 and I have a aqua-tech 10-20 gal filter and just changed the carbon filter today in hopes of getting a cleaner tank. What can I do to get this to stop? Thanks!
Give exact numbers on the ammonia and how much water are you removing? Put down that gravel vacuum!! Stop changing the carbon filter—it’s a waste of money. Many members, including myself do not use carbon. It is inactive after 7-14 days and unless you are removing meds or chemicals from the tank, it’s not doing anything.
Here's my suggestion: Put the otto and fry in the 10 gallon tank, don't worry about cycling it and continue to feed, vacuum and do PWC to your heart's content. The other fish are hardier and will survive with a small amount of ammonia in the tank and the 20 gal tank will cycle. Feed daily and only do PWC when the ammonia level is above .5 ppm. Once the water parameters are where they need to be, continue letting waste build up and only vacuum once a week, but only 1/4-1/2 of the tank per week. At this point, the bacteria should be taking out the ammonia and you will not detect it. You need to let the bacteria establish in the filter (you will later use some of that bacteria to instantly cycle the little tank).
Next, you need a plan for what you are going to do with all those fry (unless the shark starts eating them). Then you need a plan for where that shark is going to live. He may be the most docile shark, but he will out grow that tank. And while I’m at in, what kind of pleco do you have? Best to deal with everything and be prepared
Follow this advice and your tank will finish cycling. I bet it’s close, but without ammonia to feed the bacteria, which gets turned into nitrite to feed other bacteria, that tank will continue to give you grief.