yes you want to be careful mucking around with the zeolite, as you currently have 0 ammonia and nitrite there is no need to do anything instantly so it's important that you fully understand the situation and possible consequences before you take any action. It sounds like you are doing your research but I'l set it out in plain english anyway just to make sure you have all bases covered.
'normal' filter media such as sponges, bio balls, ceramic noodles etc will have this beneficial bacteria growing on them which convert ammonia to nitrite and then to nitrate which is less harmful and you remove via water changes.
zeolite just sucks up the ammonia from the water until it is saturated with it, it's a chemical process not a biological one. the appeal of it is easy to understand, you don't need to spend months cycling a tank or dong daily water changes you can just add fish from day 1 and have no ammonia or nitrite spike so no danger to the fish imminently.
The problem comes because zeolite can only absorb a finite amount of ammonia, once it has removed all that it can from the water it will just stop working and needs to be replaced. For the average aquarist this then represents a big problem as the ammonia will build up and the tank will start to cycle and unless you're testing every single day you don't know when this will happen. I guess you could add an ammonia alarm to the tank but I'm unsure how accurate these actually are and you could still nuke your tank if it happened to stop working when you were away for a weekend or something like that.
So consequently we recommend that people don't use it, it's expensive, needs replacing regularly and is unstable as your tank can start cycling at any time, much better to go with the natural process of building up the bacteria by a cycle at the start of the tanks life, we recommend this is done fishlessly as it's safer for the fish but the outcome would be the same with a fish-in cycle.
So if you currently have zeolite in your filter you are at one of two points
either
1 - the zeolite is not yet saturated meaning it is soaking up ammonia but the tank may start cycling at any point
2 - the zeolite has become saturated and stopped working but the other filter media have dealt with the cycle and bacteria is present and doing it's job perfectly.
Obviously we are hoping that it's option 2 but we can't yet be sure. At some point it's definately recommended that you change out the zeolite and add in some more normal filter media like filter floss where mechanical filtration will take place and bacteria can grow. If you are in option 2 then doing this will not make any difference, you won't have a cycle and eveything will be OK but you'll have a much better long term filtration system. However if you are in option 1 then when you remove the zeolite the tank will cycle and you'll need to do daily water changes and go through all that aggro until the bacteria grow.
The tricky bit here is I can't think of a way of telling for sure which stage you are at, any ideas anyone?
Really what I would suggest is you make sure you have a complete liquid test kit asap, fully read up on fish-in cycling so you are absolutely sure you know what to do if you do get a cycle, change some media, test every day and keep your fingers crossed. At least this way you are prepared for the worst and can act immediately rather than it being sprung on you when the zeolite stops working suddenly.
I don't think there's any need for unnecessary panic, if the cartridges are several months old there's a very good chance that you are in option 2 and will be fine. But best to be prepared for the worst.
and they are getting a green tinge around the edges, algae
