hi, glad to hear things are picking up a little.... the uneaten food question is an easy one to answer - always remove it and it there's always uneaten food then cut down on the amount you are giving them.
Let me explain about Ammonia and what can cause it and it might help to answer any further things that crop up. Okay, in a brand new tank with fish in there will always be the opportunity for Ammonia and Nitrite. This is because the filter is also new and it takes a lot of time for the sponges (or what ever other type of media it has) to grow the bacteria they need to 'eat' the Ammonia and Nitrite (I use the word 'eat' as a simple form of description) As the bacteria multiply the more Ammonia they can 'eat' and convert to Nitrite. Once there is some Nitrite another form of bacteria will form and they will 'eat' the Nitrite and convert it to Nitrate. Nitrate is what we want - but not too much of it. This is where natural plants come in as natural plants 'eat' up some Nitrate. This is what we call 'cycling'.
So the things that create Ammonia ... well fish for a start ... they wee and poop just like us, they breathe just like us ... albeit underwater but you get what I mean. This process causes Ammonia in the water. Add to that that the food we feed them also creates Ammonia - especially if left to rot (or go furry as you describe) and we soon find there is a problem ... much like what you've been seeing.
In a well established tank Ammonia is dealt with by the filter and once you become more experienced in the hobby you'll be able to see when things are going wrong. You'll also be able to judge when you are over feeding the fish and won't be clearing up uneaten food. All this is something you'll learn in time ... in the mean time you got us to help ya
Now, something to bring up while I remember .... never ever follow the manufactures guidelines on the filter. The filter does not need it's sponges/cartridges/bio balls/whatever changing every three months (or what ever else they recommend) this is a money making ploy on their behalf. The only time a filter sponge needs to be changed is when it's falling apart - and even then your well advised to cram in a new sponge with the one that's falling apart to let the bacteria 'jump' onto the new one before throwing the old one away. A new sponge means no bacteria, no bacteria means Ammonia, Ammonia means ... re-cycling the whole thing and possibly sick/dead fish into the bargain!
So the sponges (or what ever you have in the filter) can just have a swish in old tank water (I put some aside in a bucket when I do my water change for this purpose) By a swish this is all they need... over clean them and ... you guessed it - we're back to Ammonia again. I only clean my filter when my outward flow is slow. This is the sign that the sponges are getting a little bit clogged and they need a squeeze in the bucket.
As Eagles has said ... if you see something within the tank that doesn't seem right the first thing to grab is your test kit, and then do a water change. Often a water change is all that's needed. Always make meds the last resort
Hopefully I've kept that easy to understand. I don't want to over load you with info as I know it's a lot to take in and can feel intimidating. If there's anything else I can help with give me a shout