heres idea behind a tank cycle .
I can see that you already have some goldfish in the tank, thats good. they will produce ammonia in the water(ammonia is very harmful to fish). that ammonia will be eaten by bacteria and turned into nitrites(nitrites are also harmful to fish, but not as bad as ammonia). then those nitrites will get eaten too and turn into nitrates(nitrates are the least harmfull to fish). to get rid of nitrates you do water changes.
heres what to test ammonia, nitrite,and nitrate.
first thing that will happen is the ammonia will get very high and then start to lower then you will see nitrite start to get high. at this point your ammonia will go to zero.once your nitrite starts to go down you will see nitrate levels increase.once your nitrite level has gone to zero your tank is cycled. you can start doing water changes now and put piranha's in.
this may take a couple weeks.