Hello,

ive got a Mikrotik Chetau router and over ethernet i get amazing speeds(150-180mbps) but ive noticed that over wifi i get 15-25mbps.

Ive tried changing from 802.11a to 802.11n and 802.11ac and tried changing bandwidths and frequencies but nothing,I have only gotten slower speeds.Im on 5ghz wifi on wlan2(wlan1 is 2.4). Does anyone have any idea for fixing this since 20mbps seems very little over wifi compared to 150mbps over ethernet.

Post your wireless settings via /export and what are the signal strength levels of the clients in the Wireless Registration Table?

Just changing from a to n to ac, without clear purpose, will not help much. It is not possible to find the good setting with just "trial and error".

Wifi works at different interface rates (ac delivering the highest rate for Mikrotik). Multistream can multiply this interface rate. All must be supported by both sides to be used.
What interface rate is used depends on the signal strength and quality (distance, walls, ...) and the noise level (other devices emitting on other channels)

In that interface rate a lot of throughput get lost by wifi overhead (50% etc), and only one device can talk at the same time. They wait for each other. Co-channel coexistence. Any slow transmitter (low interface rate) will take a large part of the available airtime. Even if your device is fast, it will be held up by any slow transmitter in the area.

To remediate slow wifi, one has to know what the major cause is. Any setup is different, and it may vary during the day.
Mikrotik has tools to find out what the working condition is. Check the interface rate (registration table) , and check the environment (scan, snooper, freq usage)

viewtopic.php?t=183101#p915424

One possible cause that may be worth investigating is to check with a Wifi Analyzer for interference from any neighbour that is using nearby or the same wifi channels/frequencies.

Abandon all hope!!!

Hook a wireless access point from ANOTHER vendor to the Mikrotik.
Disable the Mikrotik wireless.
Connect WIRELESS CLIENTS to the OTHER WIRELESS.

Thank me later.

Does anyone have any idea for fixing this since 20mbps seems very little over wifi compared to 150mbps over ethernet.

As no information at all is given on what you have set, no one can help.

If you just want one setting to start with, set the WLAN2 to "AP bridge", 5GHz-only-AC, 20/40Mz Ce, frequency 5180, SSID filled in, protocol=802.11, WPS mode=disabled, Frequency mode=regulatory domain, country filled in, installation=any, WMM support=enabled.
Do NOT use "tkip" in the security profile.

Expected data rate in one direction under good conditions = 260Mbps for 2 stream (antenna) device (like PC or tablet), 130 Mbps for single antenna device (like smartphone).

I have a very strange Wi-Fi issue. My devices all lose internet connection over Wi-Fi, but not Ethernet.

Check the log on te AP. Enhance the log with "/system logging add topics=wireless".
There is even "radar detected" and DFS enforced channel changes as possible cause.
LOG check is crucial.

check with a Wifi Analyzer for interference from any neighbour

Yes ! The Mikrotik is a possible Wifi Analyzer. Under WLAN use the tools "Scan", "Snooper", "Freq Usage". There is also the Wireless sniffer "Sniff.." in WLAN to see the details with Wireshark.