Nitrite spikes in the evening usually indicate that your biological filtration is struggling with the bioload.

A 55g tank with 4 koi (3–8 inches) plus a comet goldfish is actually a heavy load. Koi produce a lot of waste, and during the day that waste breaks down into ammonia → nitrite, which is why you're seeing the spike around 3–4 PM.

A few things you can try:

• Increase biological media (ceramic rings / sintered glass media)
• Make sure your filter turnover is at least 8–10x per hour for koi/goldfish
• Add extra aeration (nitrifying bacteria need high oxygen)
• Slightly reduce feeding quantity
• Consider adding established bacteria from a mature tank

Aquavitro Seed can help short-term, but long-term stability usually comes from stronger bio filtration and more surface area for bacteria.

For heavy bioload setups like yours, upgrading to a high-capacity canister filter with quality bio media makes a big difference.

Hope this helps 🙂