Wireless fish finder question

[HoundsNTrout]

This is a general question to anyone you has one or tried one or knows what may be wrong.  I didn't know where to post.  I just bought a wireless fish finder, mainly for back country brook trout fishing and checking my own ponds.  SO, decided to put it in a large 1000 gallon tank that is only 4 feet tall water level anyways.   Do you think such shallow depth is not allowing it to pick up fish. Can't get it to read anything but ( fairly accurate ) depth and water temps, but no fish.  Its a $60 ebay special not sure where its made. 
 

[NymphO]

Sensitivity setting ?

[lost a lure]

Any fish in the tank?

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[HoundsNTrout]

lol.  Tank has several dozen 3 to 4 pound fish yes.  I tried messing with the sensitivity and also turning off pumps, water flow, etc. .   I am now wondering if its just simply a POS.  ?  
Maybe I'll try a better known one just to compare. IDK.  Because I gotta know if the sucker works or its useless to me. I see a few other forums stating most inexpensive ones wont find fish but are good for the contour, etc.  I don't know if I buy into that.  I'm thinking the radius of the cone or whatever of the sonar is too large so its simply bouncing off the sides of tank?  Seems odd though.  
Tanks are 4' X 8' diameter. The 500G tanks fish are probably too small and possibly to dense to differentiate.  

[fisherdude]

I'm afraid you answered your own question $60 eBay special POS. Try it on a lake.

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[HoundsNTrout]

LOL. Yup. thanks fd.