I have 3 planets who are for 90% complete but even after hours of looking for that final species it seems impossible to find and it's driving me mad...
Getting 100% completion on a planet is difficult. Throw a water planet in the mix, or a low fauna planet, and it becomes that much more difficult. From my playing experience, there are some things to keep in mind;
Certain species do indeed only come out at night. On one planet, as soon as nighttime hit, I started noticing these weird pulsating plant things. They looked like that one Windows 2000/XP screensaver with the morphing spikeball.
Creatures may be really, really tiny! Ranging from babies of certain species, to actual floating garden-weed looking things, species can be small. Keep your eyes peeled, and your Analysis Visor up.
Aquatic and Flying creatures are easy to miss, as greg-449 pointed out. They can be tiny or large. For water-based species though, they tend to be large and easy to spot, or they stick in packs. They can easily be seen by using the Analysis Visor and looking for a pack of red dots. This indicates that you haven't scanned them yet, but I'll get to that later.
For Flying creatures, don't be afraid to shoot them down. If you can find the body, you can still Analyze it.
Use the Analysis Visor whenever you feel the time is right. This will tell you from a distance whether or not you have scanned a creature already. If you have, they will have a green dot on them. If you haven't, the dot will be red. As far as I can tell, using the Analysis Visor and scanning your surrounding area will also show white dots in the distance, indicating that animals are nearby. I play on PS4 on a 32 inch screen, and sometimes it can be difficult to tell what color the far away dot is, and it doesn't stick around forever. It is enough to point you in the right direction of potentially unscanned species, however.
Since patch 1.07 this no longer is possible. Planets without animals no longer have the 100% completion box and therefore no option to upload it.
Is there perhaps an easier way of exploring a planet completely?
I spent a lot of time on my starting planet and the first I visited after that. Only to find out right afterwards that some planets do not have any species and you immediately can upload the 100% completion to update your milestones. From my experience these usually are moons.
Of course this does not give any Units for reward. But making money is much easier by farming resources anyway.
To upload the 100% completion and get the Journey Milestone updated, you need to hover the pointer over the box which shows "100%" and press ╳ (on PS4), not ▢. That's the same when you want to upload the 100% for a planet which has species. You do not get the Units rewards automatically just by discovering all the ????? entries.
Sorry, german screenshot...
