It is as in the picture in real life slight yellow tinge
Okay. Slight yellow is better than no yellow.
I'll try my best to help you troubleshoot, but plz keep in mind that I really don't know the history or timeline on your system so it's simply what I would do if it were my tank...
1) buy marinepure and a bottle of bacteria. Reseed the aquarium and test frequently over the next week to see if your ammonia and nitrite disappear and if your nitrate level increases. If ammonia and nitrite go away, it was likely a mini cycle. Do you have carbon in you canister? If not, you could also add half the recommended amount to help with water quality (the calculators on the BRS website can help you work out how much to add for your system)...
2) buy an RODI system and slowly change out your water. Test your nitrates frequently since you don't want to overdo it and bottom them out. Buy a phosphate test kit as well. In a fish only system or softie system, aim for less than 10 nitrate and less than 0.05 phosphate. If you go for an LPS system, keep phosphate below 0.05 and nitrate between 2-5. If you go SPS, nitrates below 2 (high demand would be best at .5) and phosphate at .03). Keep in mind that neither should ever be at 0! If you still had ammonia and nitrite as a result of the tap water, it should be gone by the end of this stage.
3) slowly change over your media. What you have in there now (ceramic balls etc.) will work better in freshwater systems but may be counterproductive in saltwater. You'll have already added marinepure, so ever so slowly, remove small amounts of the ceramic balls etc. Keep testing nitrates! Once that media is out of your system, you can increase your carbon to the full amount. If your phosphates are high, you can add GFO (half amount at first and work your way up). Add more marinepure if needed as space is freed up. Consider converting you HOB filter into a makeshift refugium by adding a plant light and putting up a filter screen then add something like chaeto so it will naturally help with nitrate and phosphate.
4) buy a QT system. Add a couple of marine pure balls to the HOB filter and seed them with bacteria. How often you'd change the water would depend on the size of the system, what you have in it and what meds you're dosing at the time...
In the meantime, as for your poor little blenny... If he isn't eating, but the rest of the fish seem fine, it could be an internal parasite/worm. The best medication for this is prazipro. It can be added to a reef system but not one that has prime in it, so unless you buy a QT system, you wouldn't be able to dose it. Is he eating even a little bit? If so, you could try mixing his food with seachem metro and focus.