Of course, such large changes are not generally possible with a marine tank, though they work very well. This is why saltwater hobbyists often rely on nitrate-reducing systems like live rock and deep sand beds, and it is why reef aquarists usually keep only a few small fish that are lightly fed to minimize waste production.
Do a 30% water change as soon as possible
Every couple of days, do a 10%-15% , and after about a week,take a sample of your water to a fish store to get it tested. Most will test fresh water for a minimalfee, or even for free! If the store you got the fish from won't, checkto see if there is another local store that will. At this point, yourwater should test with high ammonia and maybe a trace of nitrite. Ifit isn't, don't worry. Just give the tank time. The cyclingprocess usually takes six to eight weeks.
How to remove nitrate from the aquarium | Algone
Fish waste and decaying fish food release ammonia into the water, and ammonia is severely toxic to fish. In a new fish tank, the ammonia quickly builds to dangerous levels, but is non-existent in established tanks. This is because bacteria from the genus Nitrosomonas or Nitrosococcus will grow and inhabit any environment with high ammonia levels, and they eat ammonia and remove it from the water. An ammonia-containing tank will develop a culture of ammonia-eating bacteria after a few weeks. This isn’t the end of the cycle, however. The ammonia-eating bacteria remove ammonia but convert it to nitrites, which are also toxic to fish.
Nitrites and the aquarium nitrogen cycle | Algone