MYF Tower Controller reported they issued go-around instructions to an aircraft but did not assign an altitude and the aircraft flew below the MVA.

Narrative

I was working Local 2 position; which includes working all of the aircraft vectored in for the approach. I was switched (From SoCal Approach) an Aircraft X on an approximately 5 mile final; who had been vectored in for a practice approach. The Aircraft X was showing 80 kts. over the ground. I was then switched an Aircraft Y; on an approximately 8 mile visual approach; who was showing 200+ kts. over the ground. My first reaction was to ask the Aircraft Y to slow to final approach speed; which they responded that they would. However; I still did not think that the sequence would work. I asked the Controller in Charge (CIC) in the Tower; who was also working ground control; to coordinate for me to ship the Aircraft Y back to SoCal Approach for re-vectoring. In an effort to assist SoCal Approach and the pilot; it was coordinated that the Aircraft Y would be turned right heading 360. However; I failed to hear that I needed to issue a 'maintain 030' to the pilot; and I did not do that. At the time of the switch; the Aircraft Y was at 3000 ft.; but he descended after being switched. This descent caused the Aircraft Y to go below the MVA for the area. SoCal vectored the Aircraft Y in a tight circle; pointed him back at MYF and switched them. The Aircraft Y landed safety on Runway 28R without further issue. Several things should; and will in the future; be done differently. Firstly; I never explained to the Aircraft Y why he was being re-sequenced. He was told he was following a slow Aircraft X; but when I switched the Aircraft Y back to approach; and when he returned to my frequency; I did not explain to him my actions. The aircraft deserved that explanation. Secondly; I should have not tried to 'be helpful'; and instead should have given the Aircraft Y what any other IFR off of MYF gets; which is runway heading and maintain 030. In the future; I will stick to what I know is a safe procedure for IFR aircraft. I do feel that the Aircraft Y was given to MYF at an excessive rate of speed; and any kind of sequence that would have been tried with the two aircraft would not have worked.