You mentioned placing a decoration from another tank into the one you are currently allowing to mature, or (cycle). Is the tank you removed the decoration from an established,cycled, tank? If so,, you could borrow some filter material from that tank and stick it in the filter of the new tank. This will help the bacteria in the new tank to develop a little faster. Borrowing a cup of gravel from the other tank (if cycled), and placing it in a section of nylon and pushing it into the substrate (gravel )of new tank will also help.
As others have said ,don't add any more fish until your new tank has matured or (cycled). I recommend dechlorinator such as PRIME or AMQUEL+ for use with new water that goes into the aquarium. Be sure and add the dechlorinator before you add the new water to the tank.
Leave the filters alone for the next month. IF and only WHEN they become clogged, you may simply swish the pads,cartridges,etc around in old aquarium water that you take out during water changes and stick em back in. Don't clean anything with tapwater unless the tapwater is treated in advance with dechlorinator. The chlorine or chloramines in tapwater will have negative effect on the beneficial bacteria thus,, always clean filter material with old tank water or dechlorinated water.
Feed the fish very sparingly, maybe feed them a tiny amount of flake food once ,every other day . Overfeeding them will cause elevated ammonia levels which are lethal to the fish and thus ,require frequent water changes to keep the ammonia at non lethal levels. by feeding sparingly ,,you will be doing both you and the fish a favor. Do get back to us about whether the other tank you mentioned is (cycled) or not.
Always add new water that is close to temp of tank water so fish are not shocked by sudden temperature change. Again,,, Don't add any more fish for now ,and don't overfeed the fish and this will be over before you know it. To do otherwise ,invites problems too numerous to mention.