there should be enough bacteria in the tank so no more need to dose just allow them to grow.
unless you need to do a water change (which shouldn't be done unless the nitrates get to 110ppm or higher) you don't need to add prime, prime can cause problems when cycling as it can lock the ammonia as ammonium which is harder for the bacteria to break down.
bringing the temperature up to tropical tank temperature will speed things up ie.. 25°c/78°f.
although sponge filter use air to pull water through the sponge the amount of dissolved oxygen they produce is low due to the size of the bubbles, large bubbles don't dissolve into water as easily as smaller finer bubbles although some oxygen can be absorbed at the surface due to agitation.
the ammonia measured in a test is TAN (total ammonia nitrogen) this measures NH3 and NH4 (free ammonia "toxic" and ammonium "non-toxic"), free ammonia is produced as ammonium breaks down,the rate at which ammonium breaks down depends on a few things, mainly ph and temperature, the higher the ph and temperature the faster ammonium breaks down, it can also be seen as the lower the ph and temperature the slower the break down, free ammonia that break of is the toxic deadly part of ammonia but is also the part that the bacteria consume, when your tank is topped up to 4ppm then the amount of consumable ammonia (free ammonia) is 0.105ppm (this is with ph of 7.8 and temperature of 70°f, also quite deadly) and the amount of ammonium is 3.895ppm without being consumed these levels would stay relatively stable but the bacteria consume the free ammonia making the ammonium continue to break down and the free ammonia produced continue to be consumed until the TAN is 1ppm (which is where your cycling gets stuck) at which point the free ammonia is 0.0263 and the ammonium is 0.973, the less ammonium is left the smaller the amount of free ammonia being broken of is and the longer it takes to consume.
the idea of breading such a large amount of bacteria is that it consumes the free ammonia as soon as it breaks from the ammonium speeding up the process.
the big misunderstanding that most people have is that the ammonia measured is just one ammonia and also that the filter bacteria consume all ammonia.
the bacteria require oxygen both to live and consume/covert the nitrogen compounds ie.. NH3 (free ammonia) which is one part nitrogen and three parts hydrogen has the hydrogen removed and oxygen put in its place as NO2 (nitrites) one part nitrogen two parts oxygen, now other bacteria convert the nitrites adding another oxygen making NO3 (nitrates) one part nitrogen three parts oxygen, this is why good oxygenation is so important.
by decreasing the oxygen, ph, temperature or by adding chemicals that make the ammonia safe (locking it as ammonium) it slows the process down until it gets to the point where the amount of free ammonia produced stops, the bacteria starve and die, the cycle crashes, this happens at times to established tanks called old tank syndrome.
now there are times that the ammonia will spike but the tank inhabitants seem fine and unaffected, this is due to the fact that even though the ammonia (TAN) is high the free ammonia is low, for example if the ph was 7.6, temperature is 60, ammonia (TAN) is 1ppm then the free ammonia is 0.0109 which is quite safe in a cycled tank as the bacteria will convert such a small amount quickly.
now if you increased your tank temperature whilst keeping the same ph but increasing the oxygen saturation it should speed the process up, keep topping up to 4ppm daily to increase the colony that should now be established and hopefully it shouldn't be too long now.
to find out the free-ammonia levels use this ..
Free Ammonia Calculator
to find out the safety levels use this ..
Your Guide to Ammonia Toxicity - Aquarium Advice - Aquarium Forum Community