Hi and Welcome to the beginners section adymcd!
I see you are receiving excellent advice from C101 and Mo. A wonderful hobbyist forum like TFF is a whole different place than the local fish shop. We see multiple cases like your situation every month and have grown quite used to the very different info we see the stores and many beginner books give out. Its a somewhat complicated story but the advice in many shops may have come about as a combination of business pressures and also a lack of patience on the part of shoppers/beginners that come in to the shops. Business owners know that they have only a short time span to capture the interest of shoppers and pull them in to the hobby and the purchases that will keep them engaged. Over time the selling process has separated completely from the needs of the animals themselves and in fact their deaths tend to reinforce the business model that has the feedback of higher sales if the first tank or two of fish all die. Of course, again, I'd like to emphasize that this generalization has plenty of exceptions in great shops out there that are owned and run by experienced hobbyists such as you'll find here on TFF. Anyway, now that you're here, you'll gradually see this picture emerge as you get in to the hobby.
While the weeks of fish activity and feeding will have been causing a fish-in cycle of sorts to be occurring in your filter, it remains to be seen what the real result is, since you did not have the information or prior knowledge to perform the cycling systematically. The only way to test this is to obtain a good liquid-reagent based test kit, as C101 has outlined. I too am one who recommends the API Freshwater Master Test Kit as a good one to help beginners get a feel for the real situation in their tank. Its important to realize that you are not just checking this one time to see if your tank system is ok, what you are really doing is -learning- about the single most important baseline set of knowledge for all freshwater hobbyists. It will do you in good stead for many years if you continue to keep tanks.
The other powerful baseline set of skills involves water changes and the maintenance habits that surround them. The water change habit should be established early on as a beginner. If one falls into the bad habit of simply topping up tanks to take care of evaporation, problems will eventually result. Good fish water contains all sorts of minerals and organics and when water evaporates, these are left behind. If not removed periodically, they will build up and the fish will become used to too high a level of them. So the good habit is to always -remove- water, in addition to adding the water that will top up the tank. Not only that but the removal of the water should always take place as a "gravel clean" or "substrate clean" process. Because debris is a bit heavier than water, it will always eventually make its way down to the substrate and collect there. Some of this debris will expose charged ions on a molecular scale and so some chemicals in the water which have charge properties will be attracted to it. This leads to the substrate areas being traps of a sort for all sorts of the things we want to be regularly removing from our tanks.
This is why you always see "gravel siphons" for sale in tropical fish shops. The clear cylinder end is used to "deep clean" the debris out of the gravel and help it to be siphoned off as the water is sucked out of the tank and off to your backyard garden or the drain. Out goes lots of organic debris, which if left in the tank will be broken down to eventually form ammonia and feed in to the nitrogen cycle of the tank. In addition to the organic particles (from excess fish food, fish waste and live plant debris) we also want to remove plenty of unseen things like heavy metal molecules and organic molecules of various sizes. The water can be crystal clear and still contain excess quantities of these things.
So the "weekly gravel-clean-water-change" will be a skill that the members will help you with. The return water should normally be tap water and it should be treated with a good "conditioner" to remove chlorine/chloramines. In the cases where a larger percentage of water has been removed (and this is often quite a good thing) then temperature matching should be carried out, roughly, by testing that the return water feels the same to your hand as the current tank water. This water change skill is not just for weekly maintenance, it is the same procedure that you do in times of concern or crisis. If your test kits show that ammonia or nitrite(NO2) (the two poisons we learn to keep our eye on) have gone above 0.25ppm or 0.30ppm for some reason, then the gravel-clean-water-change (often to the larger tune of 50% to 80% of the tank volume) is best action an aquarist will usually take.
All this hobbyist talk will sound like a lot at first but I promise you that eventually, if you stick with it, you'll be pleasantly surprised at the simplicity of the processes that will make you the master of any size freshwater tank your family wants to have over the years. The members here are great and usually there will be someone happy to share their thoughts and experiences.
~~waterdrop~~
edit: spelling
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