Ok if u have 2 knifes one will kill the other eventually ..... And I doubt it is a moray/snowflake eel .... But if it is .... I should ditch it or all the other fish in tank cause a moray eel sees anything as potential food when it bites it can retract its bottom jaw to tear chunks from its victim with its razor sharp teeth witch will wound any fish badly .... Also those 2 types if eels r brackish water fish .... They can live in freshwater but only for a period of time .... If they do not have brackish they will die ..... And the other fish u have can not survive in permanent brackish water ..... But I am gonna guess u prob have a peacock eel or tire track eel as they tend to never leave the sand ... I had both and got rid of both cause they were never seen lol and my personnel opinion is a 55 gal is minimum for a Senegal but like other said would go larger to keep them happy for life ..... And as far as test kits go they sell one at pet smart and petco that can check all those things for around 30$ but will last u a long time
And I will add that if u r newer to fiah keeping I wouldn't exactly go for monster fish right off because most need to be kept by experienced hands for example a Senegal is one thing but a large gar/pbass/aro/etc. is a whole new ball game