The 15 gallon is perfect. 5ppm is very reasonable. The 150 gallon has problems. If the tank has fully cycled there would be no nitrites and definitely no ammonia. The only thing that should be present is nitrates which ideally should be 10ppm or below if you have coral and invertibrates. If this is a fish only tank, 20ppm is acceptable for nitrates. Let's say the nitrates is at 40ppm. The best way to get them to 20ppm is a 50% water change. You can do 25% one day and 25% the next day. You can cut your nitrates in half simply by doing water changes of half the tanks volume. At the start of the cycling process, ammonia needa to be added to the tank. I use pure ammonia. Get the ammonia reading to 4ppm and leave it there. In about 3-4 weeks, you'll see the ammonia readings lower in concentration but you'll start seeing nitrite readings. This is still a bad stage for fish. Fish don't like ammonia and don't like nitrites. The third and final stage takes place about 4-6 weeks after the cycle process started. You'll be getting nitrate readings and hopefully you won't have any ammonia and any nitrites. If you ever have ammonia and nitrites, then the tank is not cycled. After the cycle process is complete, 30% water change should be sufficient. If the nitrates are higher, then you may need to change more water. Hope this helps. This is not the only way to cycle a tank but it's how I do it and I've not lost a marine fish in over 2 years. Good Luck.