JBFishFish,
Fish in cycling takes more time, I've had a cycle take up to 14 weeks to full cycle. there really isn't a shortcut when you are doing water changes to keep it safe and low with the fish in there, it takes longer.
Ammonia is supposed to spike asthe nitrosomas colony grows and turns it all into nitrites, and then a nitrobacter colony grows to convert nitrites to
nitrates, and nitrates spike before it zeros and becomes all nitrates.
You have Ammonia testing 0s. You have high nitrites, and you have nitrates. as long as you are creating nitrates, you are cycling still. and you should still stay the course on water changes any time the fish look distressed, or the tests are looking like it will get to high for comfort. That's part of the fish in cycle, you sacrifice time and work, to keep fish safe with them in the tank while cycling.
it can be much quicker, without fish, 2-4 weeks. but when you have fish, and you have to keep ammonia and nitrites low through water changes and not just let them build up, your colony can't grow as fast as it could if let to run without fish in it.
I think you are near the end of the cycle, the nitrites should spike for a bit and you should see nitrates building faster, and then shortly the colony will be up to size and it will all be nitrates.
It takes more time to cycle to be safe when doing it fish in, there's no way around it unless you want to lose fish. the stuff you need for the bacteria to grow is toxic to the fish.
if you keep ammonia and nitrites low, the bacteria colony grows much slower than if no fish to worry about and letting it run it's course with as much ammonia and nitrites as the bacteria wants.
there's maybe other reasons for the slowness but you didn't give you tank
water parameters.
if
pH is low, this could be slowing you cycle, if temp is low this could slow your cycle 78F is the sweet spot for bacteria reproduction AND fish metabolism in cycle. if you have fish that can run hotter, even better if you could do 82-84F.
As far as the fluval 50, I see "
ammonia remover". what is that? if that's like
Zeolite, that's meant for an emergency situation to absorb ammonia in an ammonia spike. It's not meant as a filter media, it gets used up soaking up ammonia and then does nothing once full. And with your tap water always putting in ammonia, your bacteria should be fine to handle that by now.
Do not mess with your filter until after it cycles and has time to settle in, but that doesn't need to be in there and after 8 weeks of cycle, is likely doing nothing anymore.
pH of the tank might be useful, Ideally you'd like to be 6.8-7.0-7.2 or higher really for cycling, down closer to pH6 most of the ammonia is ammonium and slows things down cycling, but I don't think that's your problem now that nitrites are building and Ammonia is zeros. I think your nitrobacter colony need a bit more time to double and double and double again to be able to take out the nitrites and convert them all to nitrates. Another week or two. I think you're close by how you described what's going on.
If you want to keep you fish alive in cycle, its going to take lots of water changes and more time. you can tweek a couple things to try and make it a bit faster maybe knock it down to 6 weeks or so.
but I don't see anything really wrong here just you are losing your patience on it and tired of waiting for it to finish, but that's how it goes when you have to keep the bad stuff low to protect the fish.