May we ask a few questions to help us know more about your tanks. We can better help you that way.
Answers to these questions for both tanks. (You've given some of the details, but more is needed)
Size of tank.
How long set up.
Number of fish and kinds of fish.
Parameters: ammonia, nitrite, nitrate, ph, gh, kh.
What kind of testing equipment do you use? Strips, or liquid testing kit.
Water change schedule: frequency and amount of water removed and replaced.
What water conditioner, to remove chlorine and chloramine.
Type of Filtration.
Is the tank heated? What temperature do you keep the tank?
Feeding schedule and type of food.
Plants and substrate.
The answers to those questions will help us be able to help you.
I think your tanks have not cycled yet. This is the process by which your tanks develop the beneficial bacteria needed to break down the ammonia into nitrites, and then the nitrites into nitrates.
Once the beneficial bacteria are in your tank, usually in the substrate, on decor and plants, but primarily in your biofilter media, the sponge-like thing you described in your filter. That part of the filter you don't want to replace or clean too vigorously. The bacteria start to grow there, but it takes time.
In the meantime, you need to do frequent water changes to keep your ammonia and then nitrites as low as possible. Certainly below .25 but at 0 is really best.
You need to do your testing of your water parameters with a liquid testing kit, such as API Master test kit. The strips are unreliable and give false readings all the time.
You won't really know what the true parameters are in your tank with strips.
A good water conditioner that is praised by most of the experienced fishkeepers here on AC is Prime. It removes chlorine and chloramines but also detoxifies the ammonia and nitrites for about 24 hours after dosing the tank.
This is HUGE!! As a noobie, I had frantic emergency water changes, often, when I discovered spikes of ammonia and/or nitrites, sometimes late at night or some other terribly inconvenient time.
With Prime, if you discover a spike you can do the emergency dose, which is 5 times the usual dose, and it will keep your fish safe until you can more conveniently do the water change, but you must do it and within 24 hours of the emergency dose.
You must test your water daily, sometimes twice a day, and respond to readings of ammonia and then nitrites with big frequent water changes, as much and as often as needed to bring the ammonia and nitrites down to 0 or close.
Your fish really suffer if you have ammonia and/or nitrites in the tank. It hits them really hard, causing stress and even suffering, and their immune systems take a beating, leaving them susceptible to infection; bacterial and/or parasitic.
Some of the things happening sound like water quality issues due to the tanks not being cycled and therefore not having the bacteria needed to breakdown the toxic byproducts of your fish peeing and pooing, and the decay of leftover food, etc.
Pristine water is needed and water changes are what will help keep your fish healthy.