Would the two external filters be enough to stabilize the new tank? Or do i have to use the old substrate itself?
Hello; Powered filters do a few things. One is simple mechanical filtration in that particles of detritus are trapped. How much filtration is needed varies, being determined by things such as the number and type of fish and how much and what foods are used.
I come from a time when air powered filters were all that I had and they were very weak when compared to todays filters. I used a water vac during water changes to siphon out some of the detritus to make up the difference. I still use such a gravel vac today. So the two filters you plan may be enough.
Another thing power filters do is flow the water across the surface of some sort of filter media. The media can be the replacable type meant to trap the detritus particles and eventually removed and replaced. That same media does a second job in that it becomes a good solid surface for the colonies ofbeneficial bacteria (bb) to live on. The bb are needed to maintain the cycle of the tank. The bb can be on many solid surfaces in the tank, but the filter media are where the flow of the tank water is sure to make contact.
Some put what they call biomedia into filter systems. This is stuff meant to have lots of surface area for the bb to live on but not intended to trap detritus particles. Things like pot scrubbies are often used. I have used glass marbles.
When you start up a new tank some source of bb needs to be introduced. You are lucky if you have an old tank already in long term use. The old tank will have bb living on the many solid surfaces of the tank. This includes the substrate as well as other things. You need some
bb loaded substrate to seed the new tank. How much old substrate is also a variable. Even a small amount will have enough bb to get things started.
The trick is getting a balance between the bb and the number of fish in a tank. This is an ongoing dynamic relationship in all tanks. If you put a hand full of old substrate in a new tank there may not be enough bb right away to eat the ammonia produced from a very large fish or a number of small fish.
Over time the bb population will increase to match the load of ammonia from the fish. This can mean some free ammonia is going to be in the water for a time untill the bb population grows. Some call this a mini cycle. By taking the substrate from the old tank this mini cycle can be very short if you get enough bb at the start.
I avoid the mini cycle in two ways. First is to add fish to a new tank one at a time over several days and at the same I put the old bb loaded substrate in the tank. This allows the bb population to adjust to the ammonia load.
For big fish I try to use a lot of bb loaded surfaces from the old tank. I like to take some filter media from the old tank and put it in the new filter as well as hang some substrate in a mesh bag.
Keep in mind that the bb from the old tank need to be put in the new tank about the same time as the fish are put in. If the bb loaded substrate is put in without fish too long they will not have any ammonia to live on and will die off.
Good luck