How We Quarantine New Medaka


When I first started importing medaka, watching one fish after another slowly go downhill was honestly heartbreaking. It is not something you get used to.

When breeders and exporters send fish out, the medaka can look perfectly healthy. And a lot of times, they actually are. But once they arrive in a completely new environment, things can change quickly.

New water, new temperature, new surroundings… that shift alone can push them the wrong way. I have seen it happen more than a few times myself.

Even when there are no visible issues at first, that does not always mean everything is fine. With the stress from temperature changes and a new environment, their immune system can weaken, and that is when parasites or infections can start to show up.

These fish go through a lot before they arrive here. Shipping, temperature changes, and handling all add stress. That stress can weaken them fast.

Over time, I changed the way I handle new arrivals. This is the process I use now.

Outdoor Setup (Natural Light)

I also try to keep my quarantine setup in natural sunlight whenever possible.

Ideally, I aim for at least around 7 hours of sunlight a day. Not too hot, and not full shade either. Somewhere balanced.

The environment the fish were raised in matters more than people think. Most of the medaka we receive are raised outdoors in Japan, where they go through all four seasons.

Because of that, I try to match that environment as closely as possible. I run my quarantine tubs outdoors with natural light instead of keeping them fully indoors.

If you want to read more about why this matters: Buying Medaka Based on Temperature

Our Quarantine Process

Step 1: Separate from Day One

As soon as the fish arrive, they go into a completely separate quarantine tank or tub.

I always use cycled, aged water for quarantine. This is a must for me. Freshly prepared water can add even more stress to fish that are already tired from shipping. And I keep them in a bare tank (to minimize any other factors), with a small air stone for gentle aeration.

  • No mixing with existing fish
  • No shared nets or tools
  • Simple setup with clean water and gentle aeration

Even if they look perfect, I never take that risk.

Step 2: Slow Temperature Adjustment

Before anything else, I take time to match the temperature. I float the bag and let it adjust slowly.

Note: I do not float breather bags the same way, since they are designed differently.

Sudden temperature changes can stress medaka more than people expect. Taking a little extra time here prevents a lot of unnecessary losses.

Step 3: Gentle Introduction

Once the temperature is close, I move the fish carefully into the quarantine tank.

At this stage, it is normal for them to look pale, quiet, or stressed. They have just gone through shipping and a major change in environment.

I try not to disturb them too much at this point. I just let them settle.

Step 4: First 10–14 Days

The first 10 days to 2 weeks are the most important. I check them daily and look for early signs like:

  • Not eating
  • Fish isolating themselves
  • Clamped fins
  • Weak or unusual swimming
  • Any visible spots or irritation

A lot of problems do not show up right away. This is usually when they start to appear.

Step 5: Feeding and Recovery

I start feeding lightly.

Once they begin eating consistently, I slowly increase feeding. Good food helps them recover, but overfeeding too early can foul the water quickly.

Keeping the water stable matters more than pushing growth.

Step 6: If Something Looks Off

If something looks off, I do not wait too long. I respond early based on what I am seeing.

  • Epsom Salt – helps with stress, swelling, and digestion
  • API General Cure – for internal parasites and common infections
  • Seachem ParaGuard – for external parasites, fungus, and bacterial issues
  • Seachem KanaPlex – for bacterial infections, especially when fish begin to weaken

I do not use everything at once. I keep it simple and act early.

Step 7: Only Move When They Are Stable

Even if they look fine after a few days, I still wait.

Once they are eating well, swimming normally, and showing no signs of stress or illness, only then do I move them into main tanks or breeding setups.

Final Thought

Quarantine is not just about preventing disease. It is about giving the fish time to recover from stress, adjust to a new environment, and build their strength back up.

After going through losses early on, this is one step I do not skip anymore. It really does make a difference.