Having trouble increasing the spawning speed of explosions

Hello, I've been trying to make a game where waves of explosions spawn (game object with a sprites, with a trigger box collider that kills the player), the box collider quickly fade away with the sprites, and then the next wave spawn next to the first one, making a linear pattern (1st level). The thing is, I got it all working perfectly, the wave goes from right to left and then left to right, but I cant seem to find a working system to increase the spawning rate. Here's my code:

 using System.Collections;
 using System.Collections.Generic;
 using UnityEngine;
 
 
 public class BoardManager : MonoBehaviour
 {
 
     public GameObject timerUI;
     public int rows = 8;
     public int columns = 8;
     public GameObject wallTiles;
     public GameObject[] floorTiles;
     public GameObject watertiles;
     public GameObject player;
     public GameObject keyPrefab;
     bool playerDead = false;
     public float levelSpeed = 0f;
     public float levelSpeedIncreaser = 0.90f;
     public float intervalSpeed = 2;
     bool waveDone = false;
     bool waveDeleteDone = false;
     bool level1Started = false;
     bool increasing = false;
     int x = 8;
     int spawnCount = 1;
 
     public Destroyobjects destroyScript;
 
     public float timer;
     public int level1StartTime = 5;
 
     GameObject key;
 
     private Transform boardHolder;
     private Transform obstaclesHolder;
 
 
     // Use this for initialization
     void Start()
     {
         boardHolder = new GameObject("Board").transform;
         obstaclesHolder = new GameObject("Obstacles").transform;
         for (int x = 0; x < columns + 2; x++)
         {
             for (int y = 0; y < rows + 2; y++)
             {
                 GameObject toInstantiate = floorTiles[Random.Range(0, floorTiles.Length)];               
                 if (x == 0 || y == 0 || x == columns + 1 || y == rows + 1)
                 {
                     toInstantiate = wallTiles;
                 }
                 GameObject instance = Instantiate(toInstantiate, new Vector3(x, y, 0), Quaternion.identity) as GameObject;
                 instance.transform.SetParent(boardHolder);
             }
         }
         Instantiate(keyPrefab, new Vector3(Random.Range(2, columns), Random.Range(2, rows), 0), Quaternion.identity);        
     }
 
     
     // Update is called once per frame
     void Update()
     {
         timer = timerUI.GetComponent<Timer>().timerf;
         if (timer >= level1StartTime && !waveDone)
         {
             waveDone = true;
             waveDeleteDone = false;
             for (int y = 1; y <= rows; y++)
             {
                 GameObject water = Instantiate(watertiles, new Vector3(x, y, 0), Quaternion.identity) as GameObject;
                 water.transform.SetParent(obstaclesHolder);
             }
 
 
 
         }
         if (!waveDeleteDone)
         {
 
             if (timer >= intervalSpeed * spawnCount + level1StartTime - levelSpeed) //My attempt at increasing the speed 
             {
                 print("condition met");
                 waveDeleteDone = true;
                 destroyScript.DestroyObjects("Water");
                 if (!increasing)
                 {
                     if (x == 1)
                     {
                         x++;
                         increasing = true;
                         levelSpeed -= 0.01f * spawnCount; // my attempt at increasing the speed
                     }
                     else
                     {
                         x--;
                     }
 
                 }
                 else
                 {
                     if (x == 8)
                     {
                         x--;
                         increasing = false;                      
                     }
                     else
                     {
                         x++;                        
                     }
                 }
 
                 spawnCount++;
                 waveDone = false;
             }
         }
     }
 }
 

There must be a problem with how I keep track of time, I would like to reset my timer (that counts seconds since awake) to zero every time a wave spawns, and then respawn one at next location with a SMALLER interval of time. How can I do that? Been workingf on that problem for a few hours... figured I'd ask here for help!

I tried reducing the interval time with this structure, but sometimes I would spawn 2 waves in a row since i'm using Seconds since awake.

Thank you for your help, i am new to programming as you probably noticed -_-